Blending a family is so hard...

My husband and I got married December of last year and I am due with our first baby together this Thursday March 17th! We have an amazing relationship. But I am having a hard time finding the right way to express the way I am feeling. His son from a previous relationship is 5 and my daughter, also from a previous relationship is 7. They used to get along great and play well until recently he refuses to be nice to her. My daughter is my only child and her real dad passed away when she was four so having my husband has been exciting and new for her. He treats her like his own and is a great role model for her (her real dad was not). 

Anyways recently my stepson has completely changed... He wants nothing to do with me or my daughter. My daughter waits for Sunday's to play with him bc that's our half of the week with him and as soon as he gets here he is mean. He won't even let me feed him or make him snacks anymore. It was never like this before and I feel his mom may be saying things to him or around him that are making him feel like he shouldn't be nice. It hurts my feelings because I don't try to be his mom but I do everything a mom would. It's hard having him be mean to my daughter and her be excited and call him her brother now. And to him were nothing. He doesn't want my daughter to have anything and gets pissed if she has anything. I have taken into consideration how it may be hard for him to not have his parents together but like I said things were not even like this before which makes me think his mom is saying stuff...

Today my daughter asked him "do you want to help me make something for my mom?" And from the corner of my eye I saw him give her a dirty look and say no. So my daughter went to the room and colored a photo of ALL of us as a family and he went and drew a picture of him, his dad and the baby. Part of me feels immature for feeling this this way so please don't judge me. I just can't take this anymore. I shouldn't dread the days my stepson come home! What should I do? I didn't put everything on here as this is long enough, thanks to anyone that reads it .