Family problems

Ok I have an older sister who has 3 kids and now that I'm pregnant with twins I feel like my mom cared more for my sister this for me. My parents live in Ukraine. When my sister was pregnant with her oldest, my mom flew back and spent 3 weeks helping her out, she also did that with the other two. When I had my first she said that she wouldn't be coming back. Now with the twins, she said that she won't be back. Why do I feel like I don't matter to her as much as my sister. I don't need the stress that comes from trying to talk to her. It's all about how I will now understand my sister more now that I will have 3 kids to. There is a 7 year age difference between us. Help! How do I deal with this.