WP Menu add class to a sibling page

Hello, It’s the first time I come ask a question here and hope get some help with this problem i’m having.

My problem is because I use two menus, only show the “parents” and one show all the children of the current page. The problem is that the parent is not really a parent, is one of the children.

What I was wondering how to do, is that when I go (in the submenu) to another of the children I want that the element (the child in the menu of the “parents”) get a class, so the person knows that the second menu is still part of the highlighted first menu.

I dont know if I’ve explained well, its a little hard to explain, hope have some help with this.

Thank you!