Sojourn has interesting ideas.

UniversalTruth - maybe if you join in the husband's "savior" efforts, then a lot of good things will happen:

- you will be in the role of another "savior" vis-a-vis the younger sister, and not in the role of a "jealous wife" that you briefly considered when you were contemplating calling her. I think the role of another savior is better.

- your husband won't be spending time with her ALONE so much - there will be you as well

- you will be supporting your husband in his crusade to "do the right thing", which is something he oughta apprecaite