AIO for my friend inviting her cousin to a festival?

Me, my friend, her sister, and her sister’s friend originally planned to go to a festival in August. Due to a change in circumstances (my friends sister’s mother in law asked if she wanted to go holiday the same weekend) my sisters friend and her friend can longer come so it’s now just me and my friend, however because we are both just below 18, we need an over 18 year old to accompany us.

Without asking me, she asked her cousin if she would come with us (I do not know her cousin very well and my friend always says a lot of bad stuff which paints a bad impression of her). I do not have nothing against her cousin, nor do I dislike her, it’s just the fact I do not know her well and wanted to go to my first concert/festival with people I do know well.

Me and my friend FaceTime and sometimes she adds her cousin to the call (asking me first if it’s okay, and I feel rude if I were to say no) and it just turns into her and her cousin talking, not involving me, even if I tried to start a conversation with her cousin.

Now a few days have passed since we planned this with my friends cousin, and now my friends sister said she can now come due to her MIL moving the dates of the weekend getaway, however we cannot go with her now as we’ve already invited her cousin and booked transport and thought it’d be rude if we were to cancel on the cousin and essentially tell her to get lost. BTW we cannot go with the sister AND the cousin as one ticket was already sold so we now only have 3.

With getting back from the festival, my mum agreed to pick us up and take us all home (about a 45 minute drive). The original plan was my friend and I stay at mine, but since inviting the cousin, my friend told me it would either be her and her cousin sleeping at mine, or only them two sleeping at hers.

For this festival, the cousin doesn’t like any of the music and she’s only going for the fun of it (fair enough) but me and my friend really love the artists and I’m just kind of upset if this is going to ruin the experience of my first concert/festival.

AIO? Any advice appreciated x