I don't really think there is anything you can or should say. It's not your family. It's up to your husband, if he wants to, or the widowed son.
If my daughter was kissing and touching her b/f inappropriately in front of family (which I find gross to watch), I would take her aside and say something, but... if it were my husband's or sister's family, I wouldn't say anything to the 'kids'. I'd probably say something to my sister, but that's it.
My mother used to get nauseous when another sister and her new guy used to do that all the time and she'd always say, "Get a room!"