Sister dislikes my partner

I find it difficult when my sister is around my partner as she belittles him if we are altogether. She will make negative comments to him to put him down in front of others and I am anxious when she is around him as she does this in family gatherings and I feel protective over him and upset with her. I am not sure what to do about this scenario and would like to be able to gain some clarity for future gatherings.
Thank You