Background
My girlfriend and I are going on a holiday in her native country, about 800km from our current city. We planned this holiday months ago and we planned to stay for 2 weeks.
Problem
Today her mother asked me out of the blue if she could come with us during the travel because she wants to come visit her relatives.
Obviously, I don't want her to come with us. This travel our travel, not hers. It's not that I don't like her, but I feel that the travel would not be the same if there was only the two of us.
Honestly, I'm quite stunned about the question: If I were the parent in this situation, I would use public means to come and visit my relatives and surely not bother my daughter and her boyfriend.
Question
How can I answer that I don't want her to come with us without offending her and in a polite way?
I don't want to be rude and I don't want to hurt her, but I feel that she passed my boundaries and I want to give a firm but polite answer.
Notes:
She asked only to travel with us, she won't stay with us during the holiday.
Asking such a thing, she is also asking me to prolong my 8 hours travel by another 50 minutes.
This is my first time I undertake such a trip, so it will be really heavy without prolonging it.
We depart during the night, not sure how she will handle it.
I'm not sure about this but my car is pretty small and I'm not sure we will be ok with all the baggage if she comes ( she will surely have her own baggage).
She asked me through her daughter and not directly.
Also, my girlfriend doesn't like the idea of traveling with her mother too.
@Astralbee I'm 21, my girlfriend is now 19 and her mother is about 50ish.
@1006a We've been dating for half a year now. I've spent a total of 2-3 lunches with her mother.
@Jesse (Hope I understood your question) We don't mind her being in our same location, we mind her coming with us. We usually have to do the 50 minutes trip from my gf's house to mine, and when we do we usually sing, joke, talk about private matters and so.. that's why we would like to be only the two of us.
@Sip The financial situation is not the best one, but she can afford it if she really wants to.
Final Edit: We went for the "car is full/too small" option and her mother seems to have accepted the answer. (Actually, after doing some thinking, the car will probably be almost overstuffed)