I completely sympathise with the fact that you are uncomfortable with this, it is taboo in your culture. But even if you confront her, I honestly doubt it would change much. She's a grown woman and the fact she's keeping it secret and hidden shows she knows completely that it is taboo already. It also shows she doesn't feel comfortable with you being aware of it. So most likely, were you to confront her she would feel her privacy was disrespected and embarrassed, maybe even shamed and controlled. It doesn't seem that this relationship is harmful to her or anyone, and depending how close they are as cousins (first, second, etc) it may not even be a problem genetically if they have kids, and considering how secretive they are, they probably realize they don't want kids, that would make things pretty obvious. Plus we have no reason to think now that they do. It's definitely strange, but... that's kind of all. They are in love, they are consenting, and mentally healthy unless you've left anything major out. So if ppl judge... beyond thinking it's weird, I don't see how they'd be justified.

I think your daughter snooping is an issue. I've read your edit and your posts. Are you 100% sure your daughter doesn't mind? Because frankly, if I had sensitive stuff like that on my phone, I would mind. And you say maybe she's been deleting the texts, well, if I had to do that so my nosy sister wouldn't read my conversations, I'd be pretty uncomfortable. Maybe she's okay with it because she thinks it it's normal since you seem to. I have a sister and that is such an obvious privacy boundary, and my sister is a gossipy 13 year old, but she knows not to cross that line. Your daughters don't seem to recognize that and that is bad. Frankly, even if she just happened upon something she should have stopped there instead of reading everything, and definitely shouldn't be telling others, exception being if her sister were in danger. It's very disrespectful.

If my sister read some of the private conversations I had with my boyfriend and told my mom, I'd be livid. I'm also "as cuddly as an angry bear" around my family and different around my boyfriend, because I'm not okay with my family knowing that side of me. And you bet there's been information discussed between my partner and I that my mom would find taboo or not like for whatever reason. But it's not her business. At all.