To be livid with my SIL

[whatismyname]

SIL very kindly looked after dc whilst I went to work for a day. We do not let them have fizzy drinks. They are 5 and 3. SIL gave them fanta and said don't tell Mummy I am livid.
She knows we don't give it to thme

Unreasonable to be livid about a fizzy drink. Reasonable to request she doesn't say 'don't tell mummy' but not in an angry accusatory way. It was probably said unthinkingly, but there are some nasty associations to telling kids to keep things secret from their parents like MrsTittleMouse picked up on. But no need to get 'livid' just say that you don't want your kids to get the idea that they should keep secrets from you about what happens when they're with other grown ups.

YABU. its fizzy pop ffs. once wont kill them