Mother In Law LOVES the ex

My partner and I have been together for nearly 4 years, live together. 
My partner has 3 kids to 2 different mum's. 
The eldest was a result of a one night stand and his Mum is completely in love with her. She invites her to all the family gathers and calls her 'daughter in law' and I'm 'girlfriend of my son'. She did this to the mum of his 2 kids as well and since they broke up she never gets invited to anything. 
I have a hard time dealing with her cause it puts me off trying to have a relationship with her when she can't let go of this 'ex'. I use the term 'ex' lightly due to the briefness of their 'relationship. 
I wouldn't have an issue if both mum's were invited to stuff but his mum is really focused on her and talks about her all the time and they have one on one dinners, etc that she feels the need to tell me about.