It is up to the person who is throwing the shower to invite you. If you don't want to go, I'd say you're off the hook.
"Not feeling well" might make it a little easier for grandma to take though.
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"If you have men who will exclude any of God’s creatures from the shelter of compassion and pity, you will have men who will deal likewise with their fellow men." — St. Francis of Assisi, 1181-1226