She makes up reasons why she can't come to family events - like she feels she inconveniencing others, or she's too sick, or it's too cold. Last week, during bitterly cold weather, she went to a baby shower with this sister. But tomorrow is my granddaughter's christening and my sister is moving and can't attend and my other sister was going to bring my mom and suddenly, today, she is unable to attend because of cold weather (it's going to be 40 degrees tomorrow!) - I don't think this is the reason!!!
Is your other sister the practical down-to-earth light-hearted type who could carry it off without embarrassment? It seems a pity for your mother to miss the christening: what about turning up and whisking her into the car willy nilly? As long as you've checked carefully that the rest of the family can take care of any personal needs, it might just work. But unless you're sure of your ground, tell your mother you understand and play along with the cold weather excuse.
Since your sister is moving, your mother may need a chance to learn to trust someone else in public situations.
My mother is like this with me. She won't leave home without me. I think it may be because I am used to handling some of the things that might happen. She is afraid to go anywhere with other people. I think she is afraid of embarrassing herself by saying or doing something.