If you're going to go ahead with this, no legal protection etc at least try and find out this guy's family medical history.
As someone who is married to a man who is a product of infidelity there has been many medical situations that could have been managed better or even prevented if his parents, him and myself would have known. There are genetic/hereditary conditions that don't show up at the start, some are triggered through puberty, early adulthood, mid-life. Find as much as you can, not just the obvious, check for mental health as well like schizophrenia & bipolar in the family medical history. This kind of legal support can last throughout adulthood, not just until 18, it's important to know not just for you but if you are going to raise this child as you're own it's important to their wellbeing as well.
(You don't need to answer here, but ask yourself) Have you spoken to the sister if your wife took a pregnancy test while she was with her or suspected anything before returning to you? Also I would ask if your wife reached out to AP first in that week, if she would vanish for long moments, if she was aware of fights or rejection. I'm guessing if you know your wife so well you know at least if her family are lying, ask the sister, read the reaction.
As others have said I don't think you know everything to the story, not that I don't believe you when you say you know your wife, I think at one point you knew her very very well but we're talking about a WS now, not the woman you married, betrayed spouses don't think like wayward spouses, we can only see the person we married not the person they now are. You read it on here all the time, BS can't even begin to think like a WS, doesn't understand how WS act like they do or how a WS are so expert at hiding things, it's one of the things that drives us crazy.
Events seem too close together not to question - your WS wants to move closer to family then all of a sudden has an PA in that area then WS runs to her sister when PA is exposed, for a period of time, but then your WS returns to you remorseful and not soon after "surprise, I'm pregnant" ... To someone who is reading so much right now about gaslighting and in heavy investigation mode IRL myself to me this reads as your WS wanting to move closer to her OM (who she has known for longer than she has said) so WS starts pushing the topic of moving. They have, knowingly and reckless unprotected sex. Then this OM has his own issues and in the end didn't want her once the affair was discovered/blown up. Then WS finds out she is pregnant and PANICS! so since the OM doesn't want her she returns to you because you're safe and she is screwed and by herself pregnant to a man who doesn't want her if you don't take her back. Again, my brain is working very jaded right now but the events you mention are so linear and close if you write your own timeline down of events, I think you should be there for any scans to help calculate dates as well, I think your WS knew about the pregnancy before the show of “finding out” with you. Did she think getting pregnant with him would make him ask her to leave you?
You say you fear her leaving but, well, is she truly someone you want around you? Your kids? Is her carrying another man’s baby while using you as a crotch because your safe healthy for the kids to witness or you to live with? IDK you come across like someone strong enough to turn the other cheek and “do the right thing” but surely being used as plan b will eat you alive one day. I can only hope you find yourself in a healthy R where you don’t question motives and authenticity of affection (because it’s hell being uncertain if they’re only with you because you’re safe.)
Anyway, forgive me, I went on a ramble, my own demons leeched in, forgive me. All I wanted to say, incase you haven't considered it yet, is to find out about this OM medical family history so you're prepared for the future because not all hereditary issues shows up in gestation and early childhood and trust me you want to be prepared.
(again, no need to answer) but have you told the OBS about the child as well? I think it's kindness to let them know.
[This message edited by LostInHisFog at 11:17 AM, April 27th (Tuesday)]