In the sixties my sister and I had a sexual relationship for about 2 years in our teens. It started with showing parts and developed into masturbating then eventually into occasional protected s**. (I worked weekends in the local pharmacy shop and had access to condoms - no machines in those days.)
It was always mutual and usually instigated when I made a comment such as "are you in the mood"? and she would say yes or no. Eventually she got another boyfriend another boyfriend and I was going to university so it stopped. Since then we have always been close and have been successful and well adjusted in our personal, family and professional lives. I don't think that just be cause we were siblings that made our consensual, protected enjoyable sexual relationship wrong or harmful.