i think you should do what you normally would do. If the bride is too weak to dance, she will probably enjoy watching her guests smiling and enjoying her day. As for songs about being together.. unless someone has said for you to watch-it in regards to those songs... I would try not to worry about it. Your stress will rub off on the atmosphere. They're having a wedding with a DJ to have a happy day and not think about the bad stuff.
Without listening to the words of every song I play; seems like most are about love and having met 'that special person'... if it says something like "we'll be together forever"... well, they will be and that's ok. "Grow old with Me" might be one I would avoid, but the rest should be fine.
This is a wedding, a union... not a funeral...no sad allowed on your part
I would bet they'd just like to try to forget cancer for a day. Now, if someone gives a toast and says something... that is beyond your control.... you have to go with the flow. Be happy, that's what they want.
Good Luck.. I'm sure you'll do fine. I feel for you though.... it's hard for us not to get emotionally involved sometimes. It's a wedding day.. a happy day.. smile
and know that you're doing something wonderful for the bride & groom and their families.