My niece looks like me and nothing like her mom. Everyone would point it out and the mom hated it. She would get mad at me and make a point to say loudly that niece was her daughter for any strangers in the vicinity. I never said anything. It's not my fault we looked alike. As far as I was concerned, the moms feelings weren't my responsibility. I wish the mom could have let it go.

My advice is to let it go. Your sil is just trying to get under your skin because she's jealous and her feelings are not your responsibility. Let her be jealous. She's only making herself look like a fool. Don't make yourself look like a fool, too, by losing your cool. Just ignore her. She's not worth it.