My husband's sister will be jealous when she sees that we are on good terms. Why?

This is normal. This kind of jealousy is that the feelings between brothers and sisters are very good. As a third party, you intervene in the feelings of their brothers and sisters, and at the same time you occupy your younger brother. When you are a sister, you will definitely lose your mind. If you have sex in front of your sister, of course she will be jealous. I don't want your's feelings to exceed those of their brothers and sisters. This situation will gradually improve with the passage of time, and families will live their own lives.