What an awkward situation! I never understood why one person's needs throw off everybody else's plans. You and your dad want to celebrate with grandma. Why does this unrelated person have so much control?

I have had similar situations with my in-laws. If there is any conflict whatsoever with my brother-in-law, my mother-in-law and my father-in-law want the plans changed. Sometimes the brother-in-law changes his plans after the original plans are made!

I hope you get this worked out.

Gary