I'm upset with my fiance's parents.
Is there a disagreement behind the situation?
They are so inattentive. They refuse to do things they promised for my fiance's birthday.
You should let them know how frustrated you are with them. If they continue, you might just have to make your own plans regardless of what they do.
I'm thinking about getting rid of them from the picture.
Well if they aren't abusive that's not really an option. But it's okay to express your disappointment with them.