My boyfriend is best friends with his ex: Advice?

My boyfriend is best friends with his ex, and it makes me extremely uncomfortable. I do trust him. I just do not like the situation. I have expressed myself on several occasions about this, and he says that it will not end because nothing is wrong with the situation. I kinda feel like the third wheel. They talk all the time and see each other almost every day. The family things that we should be doing together take place with her and her fam. I’m usually invited, but it is just too uncomfortable for me. Any advice??