Stepchildren and pregnancy

I've been a part of my SO's daughters life since she was 2 (now 9) and I had a very hard time when I was pregnant with my first child...for whatever reason everything she did made me angry and I had a hard time being around her...eventually a few months after my son was born it went away. I'm now pregnant with my second and feeling the same way all over again, except add in her preteen hormones and sass and the fact that we're living with my MIL in a tiny 2 bedroom house until we find a new place and I feel like I'm going to explode. Since its summer I watch her while my SO and his ex both work...but tonight finding out I'm going to have her overnight the next two weeks brought on a full blown panic attack. I hate that I feel this way about her while pregnant and I wish I could just be happy. Idk if maybe it's some motherly instinct to protect me and my own offspring but it sucks and it is definitely making this pregnancy more difficult. Do any other moms struggle with this?