Divorced grandparent being childish

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My parents have been divorced for 7 1/2 years and separated for 8 1/2, both have moved on with their lives however, my mother refuses to even ecknowledge my father if he passes her in his car and waves, which that's fine by her choice. Now for the problem, for the past 7 years my sister and I have had to have 2 separate birthday parties for each of our children because our mother refuses to be in the same room with our father. That means alot of $$$ that we shell out and the hassel we go through to accomidate our mother. But here is the kicker, for the first 5 years of their divorce my mother has not gone to any of my son's birthday parties. Every year she gives the same lame excuse of "Well you know it's kind of hard to go and have to hurry back to go to work you know." (She owns her own restuarant and bar which means she's open 7 days a week & Saturdays only opening at 4pm, and I live 35 minutes away.) So for the 5 years I've had 2 parties to accomidate my mother only to have her and the rest of my moms side of the family to not show up and each year my son cries and oviously I take it personal. (My family only travels in packs like wolves, so that would mean my mom is the Alfa female, why,,, I don't know???) So year 6 I decided to go to them and have my son's party in their domain but they wanted to combine his cousin's birthday party together which in theory that would be great, however the party was more focused on his cousin and my son was silently sad. (His cousin is a older girl if that make a difference.) Now year 7 is here and I need HELP, if anyone has any advice that is going or has gone through this,,, I could use it. And don't get me wrong I did write this in hurt, bitterness and a touch of anger. Maybe I'm wrong to feel this way. Help