I'm going to tell my girlfriend that my family will always come before her and our future together
Will she agree to that?
No, she will leave me over it.
That seems like a weird thing to say to your girlfriend. If she's a long term partner, she's basically your family. Sounds like you are trying to end the relationship.
Nah, I just want to make sure she knows what's important.
That sounds mean spirited for the sake of being mean. I don't think this is a good idea unless you don't want to be in the relationship.