I don't want to make a cake for this bride as she is getting her cupcakes made by another bakery. I don't want to be liable if someone gets sick. How could I word that so it doesn't sound like I am being a (you know what)?

I'm very sorry Ms. ____ I've checked my schedule and it turns out I'm not available for the date you have requested.

I saw this in another thread from someone's contract. Maybe it can help.

No cake from another creator is permitted to be served at the event where my cake is being served. This is to protect my reputation. I've worked hard to have a reputation of having a high quality product. I do not know the quality of the other vendor's cake and cannot risk being mistaken as the creator of the other cake(s). It is also a liability issue. I do not know the sanitary conditions in which the other cakes are made, and if, God forbid, food poisoning should occur, I don't want to be associated with that. Violation of this policy could mean forfeiture of the cake and all monies paid at no compensation to the purchaser.

I would let her know that having cake from two sources is against you policy.

What jenlg said would work. Just maybe send this to her or call her and say that. It isn't mean sounding and this way you don't have worry about it.