^^ That's a bit extreme, any sane person would think the situation is all a bit odd and be worried, especially with a family member being involved. Sorry, Adhsur, but normal relationships aren't like that and the OP has the right to be worried. They haven't interfered (yet) anyway.
OP, I think becca's advice is good and that you should follow that. Otherwise keep tabs on the situation and don't say anything unless it becomes more serious. This may well just be a phase that will pass in time anyway.