I'm going to tell my girlfriend that her sister is unattractive and looks like a mom
This is surely a joke? Why do you want to say that she looks like a mom?
She is so not good looking.
Even though you may think your girlfriend's sister is not very attractive you should not say anything to your girlfriend about it. You are being rude to her and her family.
Yeah but I'm going to tell it anyway
I believe you should just keep that to yourself. Wouldn't you want to have a great relationship with their family? How about complimenting her instead, perhaps you will come to see she is truly beautiful.