Re: Do I have a say or not?
Since your SIL is not an adult and you are ok with causing tension with your husband’s family over this issue…then your ONLY option is to have a discussion with your FIL/MIL. I imagine your SIL (who is not an adult) will not be able to invite her friend if your MIL/FIL tell her not to. So forget your husband or the SIL…your FIL/MIL are the ones paying for this function and have the final say so go talk to them. And if you’re not willing to talk directly to the people who are in charge of this function, then let it go.