Wow!! You accept a mighty heavy burden by making yourself responsible for everyone's happiness and contentment in an impossible situation. Because it is impossible, your ultimate accomplishment will be to make yourself exceedingly stressed and still have everyone angry and/or disappointed anyway. You honor your family enormously by considering the feelings of all and desiring to please them as an act of love. But--you do not honor them, nor do they honor you, by demanding impossible obligations as evidence of love. Why not opt instead to do the reasonable rather than impossible.
You already had plans in place that were lovingly made on behalf of those you love and honor. They in turn made plans adjusted schedules to share the event celebrating a loved one. Because someone else's plans were interrupted, this does not demand you now disregard the commitment others have made to attend the celebration you have planned. To honor your loved one, you have agreed to modify your plans--a loving and reasonable act on your part--but to change them altogether does not mean merely altering your schedule, but that also of others who wish to honor your son and be honored as your guests. For demands to be made that you change the entire event on behalf of another is to demand that you place a higher priority on this individual than those on the other side of the family. This is an unreasonable, unfair and more importantly, an unloving demand. Your decision to spend time at the one party before (or after) attending the other is perfectly fair and LOVING to all. If this does not work, you could continue with your party as planned and have a celebration on a different day for members of the other side of the family. This does not seem the best solution but if those you love are honored in this way, it could work. If neither of these work, then you are back in a place where you have accepted a burden that is placed on you absent love or reason. Out of honor and love, you can be patient with such unhealthy attitudes, but you do not have to nurture them. Good luck!!