Dentent,
I don't think you said anything wrong. In fact, I think that what you said was rather gentle... "I am sorry for you and the baby that it didn't work out".
I am sorry for your SIL and her baby too. they are missing out on the best thing about motherhood. I can't imagine being a mother without BF. The sweet, loving relationship that BF fosters is uncomprehensible to someone who has not experienced it.
That said, it is so sad that your SIL didn't want help to make that relationship happen. Speed bumps happen to all new moms. Boy did I hit a few, but I wanted so badly to BF and I knew that it would be hard at first. I know it's supposed to be natural, but really, it is learned art. I wish more women understood this, then maybe they wouldn't give up so soon. Really, you wouldn't expect to master a new skill, like, say bike riding or swimming (if you'd never done them) in just a few days, would you? BF too is a learned skill and should be approached the same way.
You gave her the facts. You offered help. Sadly, maybe you are right and she really wanted an excuse to stop. How sad for both of them. But you did nothing wrong.