I would say forget them - do what you like, as you like, when you like. None of the events are on the same dates so you don’t have to worry about ppl choosing btwn weddings, except out of town guests. But if they helped arrange this rishta as a guardian to the boy, then pulled back and didn’t help out, AND they knew the dates well in advance and still booked their daughter’s wedding near your dates, doesn’t seem to me like they care to be reasonable or work with you so both events are great. I wouldn’t get it mad it… you never know how that might affect your relationship with them in the future, and you don’t want to offend hubby-to-be.
If they are worried about clashing dresses, tell them to change theirs as it doesn’t bother you!