<human>: My grandma always favored my cousins over me and my sister "Sarah". I'd like to say I'm surprised by my grandma's actions, but I'm not really. 

My grandma had been told awhile ago when Sarah's high school graduation was. The plan was that I would pick up grandma, take her to our house, have an early dinner, then all go to the graduation together. 

I decided I should call grandma and remind her of the plan, since it is coming up soon. When I reminded her of the plan, she said she couldn't come to dinner because she was volunteering at an event at the senior center. She asked me to come get her after, but I said we wouldn't have time, and that the only way she could come is if she had dinner with us. I told her to tell the senior center that she "forgot" the graduation was that same day, and that they would understand. She then asked me what was for dinner. I guess she didn't like the answer, because she said she would have a sandwich at her house, and that I could get her after dinner was over. I again said that we wouldn't have time for that, she needed to eat with us. She kept making excuses, and i got fed up, told her that the senior center wouldn't be angry *when* she told them that she had a graduation to go to, and said that Sarah would be very disappointed if grandma wasn't at the graduation.  

I'm worried that I'll show up at her house and find that she went to the senior center after all. Besides kidnapping an old lady in front of tons of witnesses, do you have any advice on how to convince grandma to leave the senior center and come to the graduation? Normally I'd ask my parents, but they have been fighting about her (she's one of the only things they consistently fight about) so I'd rather not make that worse. Thanks!

Can you condense the text into three sentences or less?
<bot>: Grandma is making excuses to skip sisters graduation. I'm worried she will try to skip it when I show up to pick her up.