yes it is. it’s not ok especially
I can understand how you might believe that. And in such a situation as I had described, with siblings of relatively close ages, the purposeful relaxation of privacy by one sibling in order to encourage the curiosity of the other could be seen in that light, BUT, the sibling is not being groomed toward one type of sexuality or another, merely being provided the opportunity to feel free to express their burgeoning sexual desires and growing awareness to their closest companion -- their sibling or cousin; presenting them with opportunities to explore, should they wish to, not steering them toward any particular sexual behavior. Yes, there is a certain amount of sexual gratification for the initiating sibling, but we should allow that the sibling who is being offered the opportunity to explore their new sensations will also benefit. They will have a window opened on their emerging sexuality which will keep them from having to enter blindly into the onset of their soon-to-arrive sexual maturity.
I masturbated with my older sister often from the age of 13 - 15 until she moved out and got married. Never again after that, but, at 62, I still jack off to those memories.
I'm sure those memories remain vivid, since they were such an intimate part of your sexual growth. I'm you (both) got to share that wonder.