My Sister Makes My Husband Uncomfortable

I don't really know what to do in this situation. I have a sister who has been flirting non stop with my husband. He has told me it makes him super uncomfortable but he wants ME to say something. He's very shy and unconfrontational and has social anxiety. I just feel if he truly loves me he will not let his anxiety get in the way of telling her to back the fuck up... Him not saying anything when she flirts with him to me is him showing he may like her too.