Would you consider a sister sitting on her brother's lap normal?

My boyfriend's sister just posted a picture on her FB in which she is literally sitting in his lap.
I din't ask my boyfriend yet. I don't know what to say if he'll ask me what part of that picture was so weird.

Do you guys think it's weird or do some families are super comfortable around each other?
Would you consider a sister sitting on her brother's lap normal?
Post Opinion