My Brother is a Mormon and a church elder with LDS. I have visited his home and family for the holidays and found no conflicts. As someone else mentioned, outside of Utah, the rules aren't as strict.

He has never impossed rules against anyone from the rest of our family having a drink or many of the other rules he chooses to live by.

Very tollerant as most of the other Mormons I have met in life.

But the "just ask" part will probably be the best, because they will appreciate that you care enough to ask, and that allows them to be free to tell you. After that, the guessing is over and everyone can get on with the fun, food and football.