L.M.
Your husband is also your girls' other parent since he has married you. Even though especially for your 21 yr old they may not have a father daughter relationship, he is her step-father and from what you described, their only father.
Your husband should tell his parents that both him and you want to be with your three kids on Christmas. You would love to visit them as well but will not exclude any of your kids to do so...that is not negotiable. If your older girls are welcome (and treated as such) you can all visit as a family unit. If not, you will need to make other arrangements.
He should also tell his sister that he is married and does not have any interest in contacting his ex (assuming they do not have children together that would require contact). Sounds like he should also tell his sister that you are his wife and while she doesn't have to like you, she does need to show you respect.
As for the visits, the two of you should let her know that an occasional "meet me at the auction" may be ok, you wish to have some visits in your home or theirs. Tell them you want to have a relationship with them and want your daughter to as well. Beware, not all parents are "family oriented" so this may never change and you shouldn't take it personally.