So, the way I see it, there's a few possibilities.
A: Susan likes you, but can't tell you for whatever reason.
B: Susan is possessive of both you and her other friend (or just one of the two), doesn't want to lose the time and attention either of you give her.
C: Susan knows both of you, and knows that it just wouldn't work out, and is trying to save you the trouble down the road.
... but, instead of going around it all like that, I would honestly just ask Susan why she doesn't want you to date Elise. Chances are, she could have a very good reason. The best way to understand other people is, surprisingly, talking to them.