Okay, so after reading through all the posts, it seems like the consensus on this matter is basically split 50/50. Now, since that is the case, I think this should come down to how the bride is going to be about the situation.
Clearly, based on your post, you are not okay with it, and I think that is a valid emotion! I can’t even imagine how terrible it would feel for your, and even though it seems nice to “not be selfish” and to “try and be the bigger person”, I think there are times when people have the right to be a little bit selfish.
Considering the extent of the pain that this would cause you on a day that is supposed to be about joy and celebration, I think that your Future Mother-In-Law should understand that this dance is NOT going to be okay. I understand that it might hurt her a little to not be able to dance with her son, but it would hurt you SO MUCH MORE to have her dance with him. It also does matter because it will sort of be bringing attention to the fact that your father is not there for a lot of the guests, and I hate to even mention this, but I imagine a lot of your guests would also wonder why they would do their dance when you’re not able to do yours. It will make your Future Mother-In-Law look selfish, whereas you just look like you’re grieving if you say that you just could not bear the pain.
I think that in these situations where the opinions are split, the only “tie-breaker” is your own feelings and opinion. Even the DJ suggested this, and since he’s been to a lot of weddings before (at least, that’s what I would imagine), he’s probably giving you the best advice he could give based on his experiences with these sort of situations.
Either way, I am so sorry for your loss, but no matter what, don’t feel terrible about your decision. I think it’s a good thing that you are being really honest about how things make you feel right now, and anyone who would say you should be okay with it is basing it on the fact that the situation would be different for THEM for different reasons (e.g., they might not have been that close to their father, they might not have lost anybody close to them, they might be a mother with a son they’re afraid to lose, etc.)