Is it a halacha or only a minhag that the persons leading the Chattan and the Challah to the Chuppah, should be still married originally, and not in a 2nd marriage?
For example, if one of the parents has passed away and the remaining parent has re-married, can the remarried parents lead the couple to the Chuppah?
It is definitely not a halacha, at best it is a minhag that some people are noheg. There are a number of seforim that try to explain reasons for this minhag, and most agree that it isn’t known exactly where it comes from.
For those that do have this minhag, there are a number of exceptions brought in the seforim. 1. If the child was born or even raised by the couple they may walk him to the chuppah. 2. if the couple is walking down a zivug sheini couple to the chuppah. 3. if it is going to cause a fight of ill feelings, then the couple may walk them down. The reason given is because the reason for this minhag is that it isn’t a good siman, but a lack of shalom is a worse siman, and ,making shalom is the best siman, as it says that the biggest cause of bracha is shalom. Therefore if by letting the couple be the escort to the chuppah is going to make shalom that is what should be done.
Note: There is an alternative option brought in Beer Moshe for the couple to walk down until the chuppah and a zivig rishon couple to continue from there.
Shulchan Haezer 2- pg. 71, Lehoros Nossan 2-98, Minchas Yitzchok 5-80, Beer Moshe 3-184, Yismach Lev 1 107, Chelkas Yaakov 3-76.