I’m spiraling ! I’m young and need advice ..

I feel like I’m stuck in a hard place and I’m Afraid to walk away and hurting my fiancé. He lost his parents at 8 years old and moved to a city all alone. He met his child’s mother and they were together for 8 years. He considers her his best Friend . He also considers her and the kids who are not his his family. She comes to every one of our cookouts or parties. They talk everyday not about the kid. Although he is open about their friendship and doesn’t hide it from me it’s still uncomfortable. We were talking about witnesses at our marriage and he wants her to be a witness. I know he doesn’t really have family and has know her for a while but I don’t think I want to marry them both. Am I being insecure? I’m 23 and my partner is 14 years older than me.