What to say if my sister wants my BAD niece to stay with me during vacation?

So me, all my sisters, their boyfriends, my cousin, and my boyfriend plan on a vacation to Myrtle Beach this year.

My sister (let’s call her Mel), her husband, and two children will be sharing a room.

Me, my sister, my cousin, and bf will be sharing a room.

My other sister (let’s call her Tessy), her boyfriend that she lives with, and her 10 year old daughter will be coming too.

Tessy asked me, my bf, my cousin, and sister if her daughter could stay with us on vacation in the room.

My first reaction was, “WHAT THE HECK?? NO!!” You see, my 10 year old niece NEVER listens. She has no respect for older adults, she thinks she knows everything, she PURPOSELY annoys people for the fun of it. She’ll sing loud in your ear, get in your face, keep poking, etc.

I don’t know what to say…I can’t say no to my sis. I think she wants “alone” time with her boyfriend . I can’t believe she put this on us to “pawn” her daughter off to us!! I dont’ want to feel like I’m babysitting on vacation >_<