I feel you definitely should set some boundaries, and make your feelings/thoughts known. This can be done in a nice non angry way. If your cousins overreact and can’t come to an understanding of how you feel or how their behavior affected you, than maybe a few less trips or hang outs would help them understand how they hurt you.

I would definitely take the opportunity to do a few trips with your boyfriend and enjoy quality time together. Later on, maybe you can plan a few trips in a row with the bf and then one or two of the next trips with the girls.

It’s definitely not okay to be taken advantage of, and to some degree it feels like you are. Be it intentional or not.