I have privately messaged somebody on facebook?

[Georgie]

I privately messaged my brother-in-law with regard to his Mother (who in fact is the Mother-in-law from hell) basically venting my feelings and frustration after many years of bullying and telling him exactly what I think of her - which was not pleasant - now after calming down I am wondering can this be deemed as slander/libel here in the UK - again it was privately messaged and not on his actual facebook wall - serious answers only please - many thanks