My bf sleeping with his foster mom?

So I’m 16 and I’m dating this guy I go to school with, also 16. I’ve known him for a long time, we go to a pretty small school. But last year he started living with his friend and his parents. From what he’s told me, his mom got into some trouble and is in prison, he doesn’t have any other stable family so his best friends parents offered to take him in and he’s been living there ever since. They seem like nice people, I know his friend and he’s a really nice guy. Buuut, I’ve noticed some weird things. I’ve been over to their house a few times and every time I’ve been there, I felt like the mom was just watching me really hard core, like every time I looked over at her, she was looking at me and seemed like she was giving me dirty looks, I felt like she never took her eyes off me. I also felt some flirty vibes from her to my bf. I told my mom and she said she was probably just keeping an eye on us because we’re teenagers that are dating and maybe just has an RBF. And that she was probably just being nice and bubbly-but she didn’t seem like a bubbly person. I was like okay, that makes since and just forgot about it. BUT, something else has happened. The last day of school was Wednesday and their whole family except for the dad left Friday for a road trip to California. I should also mention that the dad is not home a lot. And is some kind of traveling welder, idk what that means but that’s what I was told. Well my bf and I have been texting each other since they left, and Friday night they stopped at a hotel for the night and I asked him if he could FaceTime me and he told me no because he’s sleeping in bed with her because there’s not enough room and it’s be “awkward”. And I was just like “uhhh, okay? That seems a little weird but whatever” and he just goes “How is it weird? I sleep in bed with her all the time, she gets scared and lonely when her husband isn’t home”. I’m sorry, what? That’s fucking weirdddd. I told him that was really weird and that it made me uncomfortable and it just caused a fight and we haven’t talked since that night. I’ve tried texting him but he hasn’t replied. She has 4 bio kids, 2 teenagers and 2 young kids, like 6 and 8 I think. Wouldn’t it make more sense for her to sleep in bed with her small kids if she was lonely and if there wasn’t enough room in the hotel room? I showed my mom that texts and she also thinks it’s weird and it’s creeped out by it. I have a brother and 3 step brothers around my age, their friends come over ALL THE TIME. My mom said that’d be like her asking one of their friends to sleep in bed with her and that’s it’s super weird and seems like she’s grooming him and that’s she gonna call someone about it. Im gonna be honest, I want to punch her in the face. I’d be lying if I said I wasn’t jealous. But my moms says it’s not his fault. But it also makes me feel bad. He doesn’t have anywhere else to go and his mom won’t be out of prison for another 2 years.