What's sexually appropriate between siblings/parents?

Nothing sexual is appropriate. Why do you ask?

Brothers will talk about it. Parents will explain it, in an education manner.

There is no sexual behavior that is proper.

This is really a long and complicated subject best found in literature already written. Google sexual experimentation in children. Most siblings see each others' bodies and ask about differences, and maybe play doctor, but specific genital touching needs to be gently prohibited, with simple explanation. Children will ask about mother's breasts and father's genitals, but that should be the end of anything sexual with parents. They key to it all is to not get upset when you see children experimenting, or if children barge in to the bedroom when you are having sex.

Please give context?

Your question can range from, explaining sex to your kids. To sexual interference. There is a line, it varies from person to person, but there are limits.

My opinion only:
Good:
Talking about it
Buying of goods for kids if asked and of appropriate age.

Bad:
Nudity of parent or child in the presence or others
Touching or sexual contact of ANY form.
Talking about it in a sexual sense instead of an informative sense.

This is a very concerning question because we don't know what you mean. As a parent or sibling, all you can do is talk and prepare the child for their sexual life. Anything beyond that is interference and is immoral and illegal.