Being rude and not babysitting???

Sunday my aunt asked me if I wouldn’t mind helping my grandma babysit her two kids. One is a few month old baby and the other is a wild toddler, so I know she could use the extra help, but it just so happens that a festival I’ve been wanting to go to with my friends is going on this Friday night. She asked me to babysit Friday and Saturday night this weekend and I’m not sure what to say. Whatever I decide I want to let her know sooner than later so that she could find someone else or work something out if need be. Whatever my response is I don’t want to be rude, so does anyone think it is rude to say I can’t because I already have plans?