Need advice/tips to fit in with my Dominican in-laws. Been with my fiance for 6 years now....?

I agree with Maria, if his family gets along with you and they know how in love the two of you are then there's really not much for you to do. I would learn spanish if I were you. It would make things easier and you'll know what they're saying. But I would learn the language without telling them or your fiance for a while that way when you're together with his family you can hear if they're saying anything bad about you :) I love being sneeky lol