omg!! im in exaclty the same position buts its been 8 years!!!!
me and my fella got together 8 years ago, he has a daughter she was 1 at the time and his mum hated me for no reason at all, basically didnt want me around!! never gave me the time of day and didnt want me near his daughter, 8 years on and we have a child ourselves who is 6 and she doesnt hardly ever bother with her, she loves his first child like her own but me and my daughter get left out of everything!!! she says nasty thing and it upsets me, i to feel betrayed when he goes round there for meals etc but there isnt much you can do, the more wound up you get the worse it gets, i have come to ignore it, its her lose at the end of the day!! and she has missed out on seeing her 2nd grandchild grow up.
just hand in there and get on with your life, dont let her ruin your relationship with your fella, thats what she wants!!!!!! just be nice as pie and happy as larry!!! she will soon see she is fighting a loosing battle!!!
and if u dont get invited just turn up anyway and be nice and pie with everyone!! honestly nothing will p*ss her off more!!! xx