SO have 2 teenagers

To make a long story short... My SO & I started dating and the kids lived with their mom. Things got serious and we discussed moving in together & having a child. And that's what exactly what happened. I got pregnant & we found us a 3 bedroom to have space. His 2 teenagers moved in because their mom got evicted and claims she had nowhere for them to go. I'm 6 months pregnant and they're driving me crazy! They're messy and don't know anything about responsibility... And 95% of our arguement are about the kids. They mom obviously don't care and SO is obligated to provide shelter. I'm starting to look for a place of my own but don't want my child to be without his dad. But the kids are really driving me insane. Idk what to do.... Someone/anyone please help