Just to let everyone know what we chose to do for my sister in law.
After consulting with my brother, no flowers – her allergies are too severe. Nothing was sent to work, at her request.
We mailed a nice card, I sent an inappropriate “someecard” to her without telling anyone else, and we invited them (brother and SIL) to a concert (Procul Harum and Jethro Tull) and we gave her a very nice Barolo (her favorite).
She sent an email thanks for the mail card and called me, laughing, about the someecard. The concert was a blast, we all had a great time.
There is a place for humor, even with milestone celebration dates. In this case the humor was very well received – but also discrete. I don't give lots of gifts, but I do think certain milestone moments are worthy of celebration and some people (like SIL) who give much more than they receive deserve the attention.
Thanks for all the feedback and advice.