How much of a babysitting commitment do you think this message would be asking of you (details inside)

Earlier this week I received this message from my step-daughter: “Can you and Dad watch the girls one night this weekend?” We don’t watch her kids regularly but try to be available when needed, so we said yes to the babysitting gig. We have never babysat overnight for her, and it never even crossed my mind that they would be sleeping over. It was hard for me to conceal my surprise when I asked when she would be picking them up and she said some time tomorrow. I don’t have an aversion to them sleeping over, Aside from the fact that when asked when they go to bed she said no later than 11 – they are 7 & 5 and will be in bed well before 11. but she was so shocked by my surprise I am wondering if I missed something in her message. Any thoughts?

FYI, DH saw the message and he didn’t realize it was a sleepover either.