"My husband wants to leave a large amount to this son and his wife, but I am opposed to this. I have other long-standing, loving relationships that I would like to recognize"

You're within your rights to do so, and quite frankly I think you ARE right to do so. It is very common, and not unreasonable, for individuals in later-life marriages to have different beneficiaries in their wills. Don't even set up a trust - do it wrong, and they may get the money anyway. Your son and his wife have made it very obvious they aren't interested in having a relationship with you, or even respecting you as his father's wife (I wonder how they feel as fundamentalists about their adulterating mother). You have no obligation to leave your assets to people with whom you have no relationship at best, and a bad one at worst, just because they're related to your husband. Leave it to those who deserve it. That's all.