> Would only have the sister's DNA if it was an ovary transplant.

Fun fact: fetal cells transmit back to the mother and can be spotted in virtually every organ afterwards - it's called "Fetomaternal cell microchimerism" [1].

It's not a far stretch to assume the transfer works also the other way around and you can detect maternal DNA in the fetus/child, but I'm not aware if there has been research around that.

[1] https://www.sciencedirect.com/science/article/pii/S138357422...

Yep, mom→fetus/child is "maternal microchimerism" and it is also widely studied (though less so than the reverse) and seemingly confirmed.