Do we have the same SIL?? LOL I have the same problem with a few in-laws and a couple of "friends" who have yet to see my 17-month old son or my new home I moved into a year ago.
Here is what I have learned and what I would do:
First, people who are secure with themselves are not mean to other people and actually want to celebrate other people's news. I bet your SIL is jealous of you in some way.
Second, is your SIL a "friend" on Facebook. If so, she knew her comment would get back to you. I would send her a message that you saw it and it hurt. I used to be a mediator at Center for Conflict Resolution, so in my training it is important to tell the other person how you feel if you are stuck on the issue. It can be done with a "just the facts" without any name calling or other drama (e.g. "I saw your posting calling us #*!!?? and it hurt.") The ball is then in her court and she cannot argue with a fact or your feelings. I would also say something about yourself being out of line with the comment, yet your comment didn't warrant that harsh of response. You are then free of the situation.
I would then delete her as a friend so you don't get caught up in her drama. I would also ask your husband to handle this with his brother since they are the ties here.
As for your MIL, I agree with the one posting that said you will always be the in-law. My MIL had breast cancer and I cooked for her almost every day, sent meals downstairs in the 2-flat we owned with her, did errands, etc. Didn't see any of her 3 daughters come over except to deliver fast food now and then (which probably helped cause the cancer). My MIL said I was more of a daughter to her than her 3 girls. We'll one of her daughters got upset at me and my MIL threw me under the bus and turned several family members against me. It was over my MIL's brother, who was a drug-addict and was in liver failure with only a year to live. I stupidly let him move in with my MIL downastairs. Shortly thereafter he started cursing at the kids for being too loud upstairs, started cursing them out with the F word,and he started exposing himself to our children.I had him removed from our property. My toxic SIL didn't want to take the druggie in around her children either, but she yelled and screamed at me, even slapped my 11 yoa, for throwing the man out. She then wrote letters to us "from a concerned neighbor" accusing my then 14 yo daughter of having sex in the basement with neighborhood boys. Being a lawyer, I took the handwritten letters to a court expert on handwriting and got it confirmed that it was the SIL who wrote the letters. My MIL refuses to believe it and actually lied in court to protect her daughter when we went to get the restraining order against the SIL.
Needless to say, I removed my MIL and all the other toxic people from my life. It has been 12 months and my husband and kids frequently say how peaceful life is now.
Finally, you have to look at why you need the SIL approval so much. It gives her power over you. I did it for 7 years, so I know.
After I didn't care anymore, 2 of my toxic SIL and my MIL were graveling for us to be part of the family again and at a recent family funeral, they were very humble and meek, unlike their obnoxious behavior in the past where they thought it was funny that I was sitting by myself. This time I was by myself because I CHOSE to be and I gave off that confidence. It was a very different attitude than in the past where I was the outcast.
There is a book on boundaries that I recommend. I will have to look for it and get you the title and author.
I had to look at myself and wonder why I even wanted these people in my life. As I tell my husband, we wish we could be one big happy family, but we have to deal with reality and what we actually have instead of what we want.
Good luck!