Funny thing is: this can happen.
Not only with trans women! But with cis women too. People with wombs.
Why? Chimeras. This is when a child in the womb absorbs its underdeveloped sibling, leading to a person with two separate sets of DNA; their own, and their sibling's.
So let's say a cis female chimera has a child. That child might not actually be her's genetically. It may have taken the DNA string of her sibling along with the DNA of the (assuming we're cishet here) father, making a child she literally gave birth to genetically her niece/nephew/niebling.
And then when the child gets tested for their DNA they could not actually be related to their own mother. Well, in a parental sense anyway. So what has happened in the past is the mother has to have tests on her body done to find the separate sibling-DNA, so she can prove the child is in fact her's.
The more you know.