The first case was all very organic. We spent a lot of time together after I graduated college because I was unemployed and she was as well as she was moving from NYC to Boston after a year of doing Americorp. So the summer of 2001 we just hung out together all the time.
The second SIL came into my life when I was in my late 20s. I like her and we get along just fine. But the relationship is different. We never spent nearly the same amount of time together. But we never had a chance. We were older, lives were more established, etc. It makes for a slower build to a relationship.
But we keep on keeping on and slowly things change from being strangers and not quite getting how to be around the other person. Eventually you have a shared history and the jokes come easier and you have a sense of their boundaries and of your own. Enjoy your soon to be SIL for who she is. Work at the relationship but don’t stress about it.