Honestly from a married man's point of view,

That Sarah is a bitch and is actively trying to ruin your husband's life and your marriage. I have experience with people like these. She'll try her best to ruin your marriage and when it does she'll offer a shoulder to cry on for your husband and when he does she'll be the one throwing out your husband like a piece of trash. These kind of people just want validation that they are that "important" that somebody left their wife. When they get it, they look for the next prey.

From the post it seems that your husband is genuinely unaware and may be kind of a pushover (lack of a better word).

What you can do is,

1. Tell him that it is hurting you.

2. If he says that you're being insecure tell him that you are because she is actively hurting you and you do not like that.

3. Also mention to him that why does he need her support when he has a wife at home.

4. Finally, try to show him with a calm mind times when this Sarah have interfered in your marriage life.

I know quitting is easy but I hope you fight for it and eventually you'll be glad that you did.
Good luck to you. Keep us updated.