Father is going to remarry

My dad just told me today that he proposed to his girlfriend. He had been with her for over a year. My mom died 2 years ago from cancer. My dads fiancé is very nice but I am having a hard time with this news. I miss my mom so much. Two months after she passed away, I found out I was pregnant. It has been really tough without my mom. I would have loved having her here to talk about being a mom. 
I am the youngest of our siblings. I am 28. My dads fiancé has two daughters with he youngest being 18. I know that he will move away closer to her family and her two daughters will live with them. I am very happy for my dad but I am having hard time with this as well as my sisters. We have talked to him the past about how we feel, but he gets very defensive. 
Does anyone have any advice that may help me get through this time? I just feel like I am going to be losing another parent because he will be with his new family. Thank you!