Going overboard when asking for help at Rehearsal Dinner

My cousin and I live in the same town but have had only minimal contact over the years. She stops by occasionally and we go to dinner at Christmas. Her son is getting married in July and she had asked my daughter and I if we could help with the wedding. She is having the rehearsal dinner at a rec center at a park in the town we live in. She emailed me the other day to tell me that she had found a person to cater the event (a friend of the groom's aunt). She said that this person does not have a lot of support to cater and said she had volunteered my daughter and I to serve the meal and my son-in-law would pour wine and serve beer and make sure no one had too much to drink. I was astounded. This is not a financial thing. She has plenty of money to either hire a qualified caterer or have it at a restaurant. I emailed her back to tell her that we would prefer not to serve food and my son-in-law would not be available as "bartender". Did I over react? I just feell like she was taking advantage of us since we are the only relatives she has to ask.