Personally I have not seen or heard of any Hmong family encouraging their younger daughter or any daughter to marry their son-in-law once his wife has passed away... First time I'm hearing it from you guys. Not sure how reliable that is. But I will somewhat take your words for it as it's possible. However, I'm thinking it's not a cultural thing, but rather it's that the two had eyes for one another sort of thing.
I have seen, and know of, a younger cousin marrying another cousin's (who died) wife. None of the brothers wanted to marry their sister-in-law, so the choice was given over to the cousins and one agreed and wanted to... I've heard of the latter, and I believe it's sort of standard that Hmong do this... but not the daughter/sister marrying the brother-in-law.