Omg at your sil gatecrashing your holiday, what did you say to her?? Did she come on the holiday with you?
It was a total shock as dh had dropped me out to airport, I thought she was there to wave me off lol but I seen her cabin bag, she kept saying she was coming with me, but I didn't believe her, I thought she was going somewhere else, her words just weren't sinking in. Even when we went through security to airside I was still not believing her, the whole relaxing before I got on the plane (I wanted to have an alcoholic drink in the bar sit back and people watch and browse shops with no kids) this never happened as it was like I was having an out of body experience and my head was so hazy I couldn't think straight. But no she was going to the same place as me.
A couple of weeks previous she had sussed out where i was going when we'd be chatting about me heading off. We weren't sitting beside each other on the plane, she did ask people to swap 
but thankfully they didn't.
She even booked same hotel, I was so upset when I arrived, rang Dh who swore he knew nothing about it. But this is the kind of thing she does off her own back. She was in my face the whole time, no pampering sessions, no guilty free lie ons or naps, it was like a touring break, trying to fit in as much as possible. I ended up speaking to her on the last night when she told me to cheer up !!!
Now if I go anywhere and someone asks me where I'm going I give the wrong hotel and with her the wrong city. She still tells people to this day how she 'saved the day' and I wasn't left on my own. She just never understood when i told her I wanted to be in my own, she thought it was weird.
I never tell anyone what happened because I'm mortified that I never told her to do one at the airport. She's very loud, very forceful, very pressurising, a person who is very hard to say no to.