Can partner be friends with ex ?

Me and partner a both female and 20 years old. We have been dating for a year. Girlfriend has the belief that one can be friends with an ex, I don’t. She only keeps contact with one ex. He is male and dated on and off. The thing is later on my girlfriend made it official she was a lesbian. They hang out playing video games and always include me. But I’m still confused is this is normal or not. They don’t make me feel left out or do anything inappropriate but I always thought before that’s staying friends isn’t normal. Furthermore, I do get jealous and my girlfriend tries to be reassuring and understanding. Yesterday was the first time they meet in person to hang out with my girlfriend’s little sister and boyfriend, even before when they dated they didn’t see each other. I was a mess I was sad and jealous because I also wished that was me because me and my girlfriend are long distance. I told my girlfriend and she was reassuring me. I trust my girlfriend and her ex, I really do. Her ex it’s such a nice guy and my girlfriend doesn’t have many friends because her parents are very strict and basically never let her out so that’s why she lost a lot of friends. That’s why I try really hard for her, I think it was the first time she was able to hangout with a friend. He really is so respectful but I can’t help feeling jealous remembering they had a past. My girlfriend tells me everything and I can’t help but wonder if this is okay or not and will this jealousy go away? (Btw sorry for my English)