Advice on future sister in law

I would just pretend you don't know/don't care. Tough love maybe, but this is one of those countless situations in wedding planning where other people hurt their OWN feelings by assuming something they shouldn't. If you never told her that you wanted her as a bridesmaid, she shouldn't have assumed that you would ask her to be one. This is only on her, not you! Don't feel obligated to give her a "title" just so she won't be upset over it. If you actually WANT to give her a role in the wedding, she could do a reading in the ceremony or pass out programs. Personally though, I wouldn't reward someone else's assumption with a role in my wedding.

I would just let it be. Your wedding isn't for over a year so if you want to add her if the friendship improves, you can. If not, you can include her in things like the bridal shower & bachelorette if you want. If you want to include her in more, you can invite her dress shopping with you or let her get her hair & makeup done the morning of the wedding with everyone else. But I wouldn't worry about any of this until 6-8 months out to see what the relationship is like then.

Is she able to do a reading?

I personally wouldn't acknowledge this at all. I was in a similar situation during my engagement where my husbands brother would call my husband saying his wife wanted to be a bridesmaid sooooooooo bad and get dressed up, etc. etc. ...even though none of this was said to me and she really had never made an effort to talk to me (in the nearly four years since we had met). It was a great surprise for me though that throughout the wedding planning we did get closer and begin to build a relationship. One way she could help out is with the bridal shower (ex: if mom and MIL are throwing it, she can help them set up). She could always participate in the ceremony similarly to other comments have mentioned, otherwise she will definitely be plenty involved attending the bridal shower, rehearsal dinner since BIL is in the wedding party, and the wedding itself!