This must be very difficult for you to manage, and i do think something has to be done to make a shift in the dynamics. It is doubtful this will get any better on its own. This boundary issue is one that your partner has to take the lead in, and then the two of you must have a very unified approach in dealing with her. As you are closer in age to her than to your partner, she may see you more as a sibling rival than your partner’s partner.
Have a talk with your partner about the issue, and the two of you craft a plan for dealing with her. But he has to take the lead and set the boundaries. Otherwise, it will be you against her and that isn’t likely to work.
Wishing you patience and peace,
Proof Positive Blog @ PsychCentral