AIBU to wonder about her intentions.

[Widerangereader]

Would you think a sister sending a very romantic, anonymous Valentine card to her newly married brother was a “joke”?
They have never got on together very well.
He has only just found out she sent it.

She sounds like a wind-up merchant, or possibly she doesn’t like her new SIL very much.
Is she still fairly teenage? Does the family have a tradition of sibling practical jokes?

They don’t have a jokey relationship at all.
She is not a young girl.

I would think she’s trying to cause trouble between the newlyweds… either way it’s very strange thing to do! How do you know the card came from his Sister?!

No, I wouldnt think it was a joke, because jokes are meant to be funny. This sounds like a deliberate attempt to stir up trouble. Pretty spiteful and nasty really.

I think that is really nasty behaviour calculated to upset her brother's new wife.

I would assume either she doesn't like the new wife or she is jealous of the newly weds happiness.

It's definitely not a joke. It is pure troublemaking
.

My mind jumped to the (very good) Wicked Little Letters film…
This is peculiar behaviour, quite vindictive and I would assume she’s lonely/jealous. Given or a few months after valentines, has she only just admitted it, or how has this come about? Was there fall-out?