Nu, so they want to hire a DJ. Are they asking her to pay for something she feel is unnecessary? If they will take on the expense, let them be. I feel she is well within her rights as a hostess to demand that the DJ not say, play or do anything she deems inappropriate.
I’m not sure that’s its so “normal” to “negotiate” with the other relatives to make sheva brachos. What does it mean negotiate? He who comes up with the most outrageous and/or lavish idea, gets to host? What will make the chassan and kallah happy? If negotiations break down, will there be no sheva brachos that day (which is ok too)? Lastly, why is it more “normal” to make a sheva brachos in a restaurant than it is to have a DJ?