Not his baby?

Sorry it’s a long post but I’m confused and ranting. And don’t know if I should mind my business

My stepson had a child last year with this girl he met at a party….) met the chick and moved her right in. Problem was she was “living in her car” and would share custody of her dogs with her ex. So she didn’t have a home and he decided to move her in without asking my husband or I. Whatever she moves in and she gets pregnant. At first my husband and I had our doubts the baby was my step sons, because she got pregnant right away, and she wanted an abortion since the beginning. She never went to the doctors and till this day she doesn’t take their son to the doctors office. She would still go to her ex’s to do dog exchanges every two weeks. So finally baby comes and we see no similarities but my step son says that the baby mom swears it’s his etc. We finally ignore it. But since they have been broken up for 6 months my husband brought it up again. Since my step son found some videos on her phone (why they broke up) I told him I didn’t want to get involved but know that her ex is white and my step son is white and look similar so idk. But I wouldn’t go behind my step sons back. My husband thinks his son is getting taken advantage of since she doesn’t work, doesn’t have a license, doesn’t have a car or anything. She lives at her parents, and my two son picks her up and does everything for her. my step son would and still pays for everything. My husband wants to do a test behind his back, since we babysit at least twice a week. But also says he doesn’t know how he would tell his son if they came out negative…Also while I lay in bed her dude date was May 21 she moved in September 12ish. I believe they met around Labor Day weekend. Idk if I’m overthinking but should I be a part of my husbands side or just not be part of it at all?

Update since I want to stay anonymous, Money that is given to my step son is from my husband and I due to him working for our business. Which I don’t care where his money goes too because he somewhat works for it. Sometimes he asks for more because of certain expenses due to her (example her glasses) which is why my husband think she is taking advantage of him because he pays for her things still not just their son. baby was also born full term. Her ex texted her during labor to tell her congrats. I know she kept in contact with him all during pregnancy due to the dog exchanges every two weeks. Also after their son was born, because she would take he baby to the dog exchanges. My husband wants to do this because at one point her ex boyfriend told my step son that the child was his not my step sons….she kept telling my step son that her ex was lying. This is my step sons first gf…..husband wants to do the test where he is related to the baby so my step son wouldn’t know if he did the test.

I honestly feel like I should stay out of this because I have a good relationship with my step son and it took years to get to this position but my husband says I should help him because what if his son is getting taken advantage off…