Can a younger adult sister sit on her older adult brother's lap just for fun and with innocent intention?

When that is done, something dirty is normally implyed, so therefore I must say no. If siblings do stuff like that with one another, that is called incest, which is discusting. People in the olden days would have children with family members and that would result in disorders and body deformations. They have the same blood, they came from within the same person, so that is very wrong.

Another PerspectiveThe question, as asked, does not indicate the age of the siblings involved. Toddlers and very young girls should feel free to sit on the lap of an older brother. Naturally, once a girl passes through puberty, propriety dictates that people act differently.

Unfortunately, there are many who have extreme views on anything that may even be remotely linked to sexual activity.