Madara is obito,nagato and kushiha father.
you don't have to read all of it just read the basically part at the bottom.
How madara uchiha is immortal
if you all remember that madara uchiha only fought hashirama to gain power and to be immortal,madara was already on hashirama level and madara himself also had special chakra and using hashirama dna would make him on the sage of six path level of fighting since both the elder son and younger son are in one body like the sage.
also madara could have died at any time for all we know madara could have died when the 3rd ninja war started because madara said he died a while after he unlocked the rinnegan for some reason but when tsunade/itachi/konan says if you have both genes you are immortal so madara uchiha himself could have been hiding in secret this whole time and could have been the one that met kisame or even itachi and manipulated the fourth mizukage.
if this was true that madara was really immortal it could explain why madara manipulated the events in his favor and how he met tobi however you look at it tobi would need madara in order to get hashirama dna no one else would have fought with hashirama besides madara, tobi has a huge peice of dna and where can he get that from maybe from madara battle with hashirama.
how obito could be madara father.
also look at this the nine tails knows the uchiha and there sharingan and can even sense people chakra signature(it's so good that naruto was able to sense kisame real presence when the samehada and kisame had identical chakra nature's),
now we both know the nine tails thought that tobi was actually madara before he revealed himself to be but the thing is that the mangekyou sharingan at that time was obito's sharingan but the fox thought it was madara so if tobi has the same chakra signature like madara then madara could have been obito's father they share it in looks also.
1. we know that madara didn't see nagato to grow up and nagato is like kakashi age or maybe older so we know at that time the real uchiha madara was not fighting but tobi was and also that ao was able to see shishu chakra signature by seeing the chakra in danzo eye so yes you can tell the chakra signature of the eye so nine tails thought it was madara since madara has that special strong chakra.
2.also if you rememebr sage mode naruto wasn't able to tell that nagato was closely related to him when nagato was pouring his own chakra in the deva path and also put a chakra rod in naruto, because possibly they are distant relatives as well(also nagato is probably more of an uzuamki then naruto because of the traits nagato took,red hair, born with he ability to sense,sealing, nagato has shown all these things unlike naruto maybe because naruto is a namizake)
how nagato is related to madara
you have to be honest tobi and madara plan both relied on a uzumaki so nagato life had to be manipulated in some way for him to join the akaksuki like sasuke and itachi.(i really think the only reason why tobi never actually fought itachi is because he knew they were related to izuna or possibly madara himself look at the resemblence of sasuke and itachi to madara and izuna and they all probably have the same power which madara has shown that they do.(which is why tobi knew that sasuke would be strong and ggain powers like madara so that's why he says i am interested in sasuke development and needs sasuke.
remember when madara was summoned with impure world resurrection he showed his rinnegan and the thing is people are shown with the abilities they died with so madara had his rinnegan, when madara died but he knew about nagato and nagato is like kakashi age so we have to know that the real madara uchiha was at least alive like 29-35 years ago(madara might look young because he is full of life since he took the younger son dna/hashirama but he could be really old we just don't know.
the thing is madara hid in secret and we also know that the uzumaki clan was destroyed and hid all over the world so madara could have met an uzumaki at this period since they spread out instead of being in there village.
also we know that tobi the masked man was able to control the mizukage which is the land of water and i think amegakure?hidden village of rain was also a part of this country and you can just tell from the climate that they are close to each other but the reason tobi was probably there was because that was suppoose to be nagato birth place and yahiko was suppose to guide him that's why yahiko was probably so protective of nagato even though nagato had the rinnegan because all this was planned for tobi/madara for the moon eye plan that was yahiko peace.
how kushina could be related to madara
1. there are different ways to restrain tailed beast but madara and tobi used chains in order to control them like in there sharingan genjutsu which kushina can do both mentally and physically with her chakra.
2.kushina is the only person with this chakra chain jutsu that can manifest her chakra to do it physically but madara and tobi can do it both since there sharingan eye power is strong and is like the same which is why the nine tails thought that tobi was actually madara but tobi was using obito's eye and at that time nagato had the rinnegan so tobi had to use obito mangekyou sharingan.
3. if it was really true that the uzumaki can seal and are good with the yang element like the younger son(body) unlike the elder son(eye power/spiritual) than if madara was her father than wouldn't kushina be able to manfiest the special chakra that she gained from madara since uzumaki are like a yang clan so the chains can be physical as well as mentally/spirtual.
4. the chain jutsu is originally from rinnegan so how did that jutsu end up in the uzumaki side of the branch they don't have the spiritual but just the life force/body of the sage.
but kushina may not be related to him this is more of a wonder.
Just read below
basically madara is suppose to be immortal he is the sage basically without the ten tails in him he has both bloodline's within his body which are the younger son and elder son, so if he is immortal than he can make a grand master plan and when you think about it all the events that happened in the past seemed like it was all planned,like tobi being in the land of the rain where nagato is born too(and also madara saw nagato before and they look alike, also that madara needed nagato for his moon eye plan) so nagato could have taken madara eye's or could have been born from them since madara has the rinnegan if nagato father is really madara.
if you are related you may have a similar signature but the thing is it is still different so you would be able to tell who it was from the clan but the nine tails couldn't and thought it was madara and everybody else thought obito's sharingan was madara's probably because obito father is madara and there sharingans are really closely related instead of madara being obito grandfather or uncle but probably his father(if madara married a uchiha then the sharingan would probably be dominant and obito would be born with a powerful sharingan).
kusina could be related to madara because of the chain jutsu and the special chakra that both madara and kushina have,they were the only one's born with special chakra that there people and clans have even admitted and the chakra chain jutsu is suppose to belong to the rinnegan(elder son side/spiritual power of the sage) but kushina somehow got it and is known to be from the uzumaki and she can use the chakra chain attack both mentally and physically to contain the nine tails but kushina is more of a wonder and might not be related to madara.
so i think nagato and obito are definately related to madara because of looks and powers and events that took place because without them the moon eye plan would not be possible nagato was a huge peice for it to work and so is obito sharingan which is somehow is mistaken to be madara sharingan shakra signature so do you really beleive that tobi/madara just waited for people to come along or do you think they really gave nagato the rinnegan and gave obito a powerful sharingan because if madara had powerful chakra then his children would probably have it as well.
this is just a theory