Blended Family Issues?

Melissia
Anyone else dealing with blending a family and adding a new little one? I'm struggling with some unexpected feelings with this pregnancy. 
My husband and I will be married a year this month. I have a 12 yo daughter with shared custody to my ex. She spends 1/2 the week with us & 1/2 with her dad. My husband has full custody of his 3 kids. His 16 yo son is severely autistic and his daughters 8 & 10 are still struggling with their mothers abandonment. She visits them but it's sporadic and causes more hurt. 
I love his kids, but honestly, I'm still very aware that they aren't mine. I'm a SAHM, so I take care of them. I was a single mom for 9 years so I'm not used to having another parent around. And if I'm totally honest there are some rare times where I resent that his kids are always around but mine isn't. 
So we're now unexpectedly having a baby girl together. We're happy and looking forward to her arrival. I'm just surprised that sometimes, I find myself feeling like she's "his" baby and not "ours". It seems like I'm not connected to her. It makes me a little sad. 
Anyone else experience any unexpected feelings because of blending?