what a yucky situation. AND you're not even married to the guy. Awesome. Well, he needs to clean up the mess he's created or risk a permanent rift between you and his family.
Frankly, you're being treated like a second class citizen because you're not married to the guy and the SIL *is* going to be married into the family and she is pulling rank and letting you know your place. You're the girlfriend her best friends ex knocked up. You are not her SIL.
Your boyfriend threw you under the bus. How dare he say anything to anyone about the conflict you had? If he goes without you, I'd take that as a sign that he's not as committed you and your child as he should be and strongly think about cutting my losses.
**Updated: Nobody, in their responses, seems to be taking the fact that you and he have a baby together. If you live together, I think that adds another layer as well. You *should* be respected as the partner of the grooms brother, but you know that means nothing in this situation. She's treating you as illegitimate and your boyfriend should either honor you or or you should move on.**