"Whom" instead of "who"

[Ludovica Peruzzi]

Hello everyone, I have a question. I just got a scorecard where my editor marked this mistake:

"her brother Lockett, who (whom) she is charged with looking after"

Now, while I know the difference between "who" and "whom", I was under the impression that "whom" was only used in extremely formal writing, and even then most people nowadays use "who" where they should use "whom" anyway. Grammarly even says:

Many people don’t use whom in casual speech or writing. Others use it only in well-established phrases such as “to whom it may concern.” Some people never use it at all. It’s not unusual, or even incorrect in many contexts, to hear sentences like these:

Who do you believe?
Who should I talk to about labeling food in the refrigerator?

Cambridge Dictionary:

We use whom to refer to people in formal styles or in writing, when the person is the object of the verb. We don’t use it very often and we use it more commonly in writing than in speaking.

Obviously the other way around (that is, using "whom" instead of "who") would count as an objective mistake, but is using "who" in this sentence really incorrect? Thanks in advance.

[Shirley Ann Riddern Labzentis]

Both are acceptable to use. I don't feel that the editor should have marked it as a mistake. I had the same thing happen to me, but I let it go because you can't fight every editor.

[Ludovica Peruzzi]

Both are acceptable to use. I don't feel that the editor should have marked it as a mistake. I had the same thing happen to me, but I let it go because you can't fight every editor.

Thanks for weighing in. I probably would have let it go as well if that had been the only thing, but I'm going to have to request a recheck for other mistakes I've been wrongfully marked down for anyway, so I might as well bring this up too.

[Nikita Wallace]

Both are correct to use. It's confusing at times.