@bellle
I can certainly understand your fears and concerns. I would have the same fears if she was my daughter, sibling, parent or friend. I believe there is a risk for those grieving to fall prey to a person with less than pure intent, though I wouldn't know if your daughter's fiance is preying on her. Or maybe both your daughter and her fiance are desperate in their situations. Marriages born mostly out of desperation are not usually ideal.

I wonder if your daughter would consider putting some of the money from her late husband into trust funds for his children. That way if the prospective marriage broke up, the new husband wouldn't stake claim to too much and her late husband's children wouldn't pay a price for their mother's possible accidental "mistake". Big gifts from you and your husband to your grandchildren could be handled in a similar way. As for your wife's stake after a possible divorce, she's an adult and has to accept some possible losses for herself. She might want to fully understand what she could possible lose in the case of a divorce, and how to protect herself sufficiently.

If your daughter's fiance doesn't approve of such things as above, that itself may be a warning sign. Frankly, if he has any money, he might consider taking the same advice.