If you were planning your wedding and decided to invite your aunts and uncle, even though you don't have a strong relationship with them, and they decided to use your photographer to take family portraits DURING the wedding party. What would you do? Would you uninvite them, scream at them, just say no, or agree? These people had to actually deal with this. Luckily, they found out that the aunt wanted to pull this off before the wedding. But even with that. Why would someone want to do that? To be SO cheap that you even have the face to ask the couple's parents to do this is insane. Is not even crazy, is far more than that. It's your wedding day, and now you have to be looking out for your aunt ot trying to pull this off at the wedding? Is crazy.
If I were in their situation, I would definitely uninvite them. They do not deserve to be at the wedding. And it would ease the couple's mind, as they don't have to be warning photographers not to start taking family photos for some distant family. I don't think any photographer that respects their job would agree to do this. But just in case, I would simply uninvite them.