L.R.
I think that it's wonderful that you take your position as a godparent so seriously; however, what molds your husband's godfather relationship with this other child is not your own attitudes about it, but the parents' interpretation of what it means to be a godparent.
Most people, while they do take the godparent role as an honor and a commmitment on some level, do not attach the deep level of time commitment (or travel) to it that you do. Again, that's wonderful, but you were also already extremely close to your godkids' parents before the kids came along -- so close you call the mom your sister.
But the parents of this other child already are well aware they are not that kind of close to you. So why are you so concerned they will have the same expectations of you that you have of yourself?
The parents may view being a godfather as just meaning that your husband is there for a baptism; that he is helpful to them in terms of giving the child religious guidance, and/or memorializiing important times in her life such as birthdays, first communion, etc. I would be very, very surprised if this family expected your family to now surrender every holiday with your own relatives and others to whom your are close, so you can spend those holidays with them.
Your husband should talk with them about what it means to them. It may be very deep for them or it just may be a matter of "We needed to have a godparent for the baptism ceremony and we would be delighted to have you there." Not "We now expect you at our house at Christmas." Even if they consider your husband a very close friend, I can't believe they now expect that of you.