I at various points in time start to notice interactions or lack there of on social media. So I do understand what you mean about noticing what your cousin's gf is and isn't doing. It's one thing to miss some, another to leave out completely on everything. It's akin to trying to make an effort with those you care about. Or not.
It's so odd how she'd bring it up, now and out of the blue.
It is suspect in my eyes.
Sounds like you are reaffirming a desire to maintain some distance. I'm not personally keen on falsities at family gatherings. It's ok to only need to interact under those circumstances and limit contact otherwise. Unfollowing is a neat little fb tool. And share settings can indeed limit an audience.