He's gonna what?

I'm babysitting New Baby and her Big Sister today. They are having a toddler stay with them this weekend (foster care), and Big Sister was talking to me about it.

Me: So I guess you'll get to boss him around, huh?
Big Sister: Yeah, he's gonna pee on us, I guess.
Me: What?
BS: He's gonna pee on us, I guess.

Oh my. I hope not. Cause I'm keeping him tomorrow morning.