I agree with the previous posters, without being in your shoes its impossible for us to tell you what to do. I have an 18 yr old but still have no idea of the complexities of your realtionships with your older kids, I do know it can be VERY difficult though!!

I guess off the top of my head I'd say go, only that if you don't develop a better relationship with her in the future then so what you wasted an evening but if you do form a better relationship in the future the fact that you didn't go to her engagement party could be a thorn forever IYKWIM??