Better get used to it. My husband's parents are divorced AND his mother is remarried. I don't know how you lucked out being the one to host all the family gatherings, for us, both of his parents host their own holiday gatherings and force my dh and his siblings to choose one over the other, OR we all just suffer a very long day of having to go to both to avoid an all out war of affections.
It is soo bad that if we (dh, siblings and their spouses and kids) are even late to the other parent's house, or too full to wolf down an entire second meal, all hell breaks out. It's a minute by minute torture for everyone because we have to walk on eggs all year round.
If the new hubby and my dh's father accidentally cross paths it's really horrible.
There is no way to placate anyone in a situation like this. No one dares to volunteer to host a party or get together. Doing so would make you enemy number one on both sides.
For us, we "go away" for the holidays...and just secretly get together with each parent at a time very removed from any holidays, birthdays etc. Sadly, for the entire extended family holidays have lost their meaning and appeal. We have worked to make our own private time at home special and try to remove ourselves from the madness as much as possble.
I know this doesn't answer the question of what to do in those "must come together" situations like a funeral or wedding where all sides MUST come together. For us, we all have to suck it up and work really hard to keep all parties apart as much as possible...which is impossible... and just expect a horrible month of anger and lashing out to ensue until they all get it out of their systems. We made the mistake of inviting "everyone" for our first child's baptism. It was absolutely horrid. When our second came along, we had a private one with no reception or anything! Boy was that the way to go!
In the case of your daughter's party...do you really need to have the entire family present? Something like that, I'd keep between me and dh and make it a private thing...just to avoid the brouhaha. The grandparents can send gifts in the mail or come on a different day on their own for a visit.
Sad isn't it?