I find it quite baffling too. It seems that a majority of people trapped in these situations lack the strength to reject them and make decisions for themselves, rather than blindly follow cultural obligations.

Since the OP lives in Spain and is a Spanish resident, it's a rather reasonable guess that she's managed to to mitigate her circumstance to one that's manageable (by putting distance between her family and herself), so kudos to her, and yet she still struggles with a rejection of the invite which is quite sad.

Disclaimer: I'm a (semi?) immigrant who's lived in India for most of his life (before coming to Canada, which was heavily dissuaded by a lot of my maternal family) so I have a sense of what it's like to reject the cultural oppression that the OP seems to face.