Irresponsible-ish Grandma

[danner]

My lacking-in-common-sense MIL shares her big house with my SIL, who had her now-fiance move in. His name is Billy. He's a nice guy, in law school, takes an interest in my children, probably idk 30 years old.

When my MIL has my 4 year old over for a sleepover though, SHE is the person I am entrusting my daughter to. SIL and Billy are welcomed to goof around with my daughter, spoil her, have fun, etc. But
It has come around through random 4 year old remarks that sometimes MIL leaves my daughter with my SIL's childless, unmarried dude when she runs to the store. I don't like it. Not one little bit.

What do you think? Does Grandma deserve an explanation for why we don't want her leaving our daughter behind?