Should I offer to pay?

So, next week I leave to go on a vacation with my ex boyfriend and his family. Yes you read “ex” correctly lol it’s a unique situation and I’ve already been told a million times all the reasons why I shouldn’t go, so save those opinions. Anyways, I have been formally invited and we have planned on me going for about 3-4 months. The trip will last 9-10 days and include a wedding and a trip to a theme park. I’ll bring my own spending money of course, but is it appropriate to offer his mom $100 or so for food etc. for the trip? Would this be weird or insulting in any way or would it be appreciated? He and I are both 19 by the way if that makes a difference, and I can totally afford to give the money. Thank you in advance!!