There may be some buried resentment that has to do with her biological mom being out of the picture and realizing your son does not have that problem. It also may be a bit of normal sibling rivalry. It also may be that you and your husband treat her or your son differently and she is picking up on an unintentional bias toward your son. It could be learned behavior from a classmate or friend who does the same. It could be a combo of all these things.
Ask yourself honestly whether the unintentional bias is a possibility? It can be something that parents do not see because they treat a child differently because they are younger, but the older child perceives this as favoritism.
Your partner "telling off" the child is NOT going to help. It will further exacerbate the problem, especially if she is acting consciously or subconsciously because she feels your son is favored.
Of course, a form of discipline should happen, but not one that puts her on the receiving end of nasty words or actions. Do you understand why? If you are wanting to curb her acting and doing "nasty" things to your son, and you discipline her with the same technique you are "modeling" that as appropriate behavior to deal with problems. You are doing the "do as I say, not do as I do" approach which is contradictory and confusing, especially to a child.
You must build empathy within her for her brother, but NOT by shaming her. By treating her with kindness and trying to figure out what is really going on to elicit this behavior.
If she sees you as her mother, and I'm guessing she does because you are the only mother she has known, then both you and your partner need to find a time where your son is not around (maybe at a friend's) and sit down and talk with her. Tell her how much your son looks up to her and loves her, and ask her why she has been treating him badly. Then, listen.
Listen to her reasons. They may make sense. They may not. If there is a kernel of truth, and your son has some things you can help coach him on, then make the promise that you will do that. He may inadvertently be pushing buttons. It certainly doesn't make her actions right, but it gives you context to understand where they are coming from.
If her reasons are just excuses but not real reasons, do NOT call her out on them, necessarily because that will just add to the issue of trying to shame her, but remind her that he is younger and still learning, and he needs a sibling that is patient and can help him.
This is the thing to remember. SHE IS ONLY 7. By contrast, she feels loads older and more mature than your son, but in reality she is very young and learning herself. Treat her with kindness. Do not let her get away with mistreating your son, but do not model bad behavior when you do it. Use natural consequences whenever possible, and pre-emptively warn "both" children. For example, if you anticipate there will be issues when you go to the park, warn both that if they cannot treat each other nicely, you will have to leave the park the first time a problem arises. This does lump them both together in experiencing the same consequences of one sibling's bad behavior, but if you do that a few times, they may learn that they have to pay consequences and will modify their behavior.
Edit: Forgot one of the most important things. CONSISTENCY. You must be consistent in your approach. You can't follow all of this advice, and then your partner still continues yelling at her. That undoes any positivity you will get from what I am recommending.