Canonizing Professor Sinistra
Before I get into a discussion, I think it'd be best if I described the plotline of my story.
First of all, it is a Snape/Sinistra that I am determind to make as canon as possible. In 1991 (Harry's first year) she becomes the new Astronomy instructor after the former teacher retires. She previously taught at Salem Witches' Institute for four years after abandoning her dreams of becoming a Astronomy researcher for the Ministry of Magic.
Anyways, my problem is trying to make Vesper Sinistra (Yes, I know that her official first name is Aurora, but Vesper sounded a lot better.) canon. We don't know much about her in the books, because she is only mentioned twice...literally.
In my story, she was a Ravenclaw, so I guess that contributes to her fascination with the complicated subject of Astronomy. Also, she is quiet and rather shy and can seem a little bitter at times.
What I want to concentrate on is getting Snape and Sinistra to communicate and (eventually) get together. They went to school with each other, though he was a little bit older, but didn't talk to each other much. The only time they really communicated during their school days was during Slug Club meetings...though, I'm not too sure if Snape would have been in the Slug Club...oh well.
I think you'd do better posting this in Skele-Gro Plot Potion. You can get all your story-layout help there. Or if this is central to developing your OC, Vesper Sinistra (we know very little about her, after all, so you can have free rein), you can post what you already have about her at Madam Pomfrey's Character Clinic. In addition, you can read up on Snape-ICness there.