I would go above "the objective" truth because another belief of mine is that there is no such thing as "objectivity"

Yes this is a very fashionable belief these days. Everyone's opinion is equal to the opinion of anyone else, there is no 'more correct' opinion, so let's say even if I have never studied mathematics I can have an opinion on an equation. In philosophy we call this 'student relativism', because most first year students have it, but after the first year almost all have abandoned it.

Certainly the observer plays a role. There's is such a thing as theory-laden observation even in science.

I understand the approach you discuss, it's the standard approach, no one would like therapists and mediators and marriage counselors if they took sides, but scientifically/philosophically I must object, because that would presuppose that no one is ever right or wrong. What would you do if the husband showed sexual interest toward the daughter, and maybe touched her in the wrong places etc.? Ah, now suddenly it's wrong! Now suddenly one must notify the authorities, because it's illegal! What I don't like about the whole counseling field is that it yields to authority and public mores and laws, instead of creating its own (hopefully objective) values like they used to do. For instance Freud tried to do this. He might have been wrong, but at least he tried to discover some objective reasons people behave in certain ways. Now they don't try to do this, instead they try to help the client, and whatever helps him feel better must by definition be the right thing to say. So I think it has become a question of money basically, of offering a service desirable to the customer, instead of seeking objective truth like they used to.

Sometimes 'objective truth' might mean just what you said: helping the client understand what is going on, listening and then offering a better way to put his thoughts and feelings into words and mental pictures and metaphors, using one's knowledge of history, literature, feminism, whatever, to say to the other person, "look, the reason you feel this is because of this" or "this thing you describe and feel is like this thing this person described in his book, and maybe you should read it, perhaps it will help you" etc. And then the partner will also be able to understand the other's perspective better, because it will be framed in more accurate words and mental pictures. So basically helping their client in their journey of self-discovery.

But I also think people use lies, self-deceptions, delusions, inconsistencies, contradictions, confirmation biases, and so on and so forth, all the time, and it would be good if therapists could indicate these things to people. Usually people think whatever it's to their advantage to think, and if they were taught not to do that, society as a whole would improve along several metrics.

Sorry for the long reply!