Inappropriate dynamic?

[Jesstoimpress]

My sister and I were discussing her boyfriend of 6 months Bob (not real name). She told me his 13 year old daughter who we'll call Sarah, will regularly walk around the house in underwear or even from bathroom to bedroom naked. The relationship between them is very physical, with Sarah climbing all over Bob, jumping on his back, playfighting. She will straddle his lap and grab his cheeks or slap his face lightly to get attention if he is watching television on talking to my sis. There is little discipline or routine. Sarah is rude and disrespectful to Bob. He will then make a sad face and say things like "Oh, Sarah be nice to poor daddy".
Bob also encourages Sarah to hop into bed with he and sis in the morning for cuddles which sis finds uncomfortable.
Bob has 50/50 custody.
He is a kind, nice guy and otherwise my sister is really happy in the relationship.
This would totally give me the ick. My sister thinks maybe because we are just not from a tactile family that we don't really get it.
AIBU to think this is not a healthy relationship between father and daughter?

I think a lot of that would make me run. Straddling him? Jumping into bed for cuddles? Just no. I think he needs to understand that at her age, this sort of behaviour is really unusual. I remember a few years back, one of my year 11 students sat on her dad’s lap during one chat with a teacher. They are not from the U.K., but a not too distant European co7ntry. Mum was there too. We all thought it was a little odd.

Erm ... no. Just no. And we're a pretty relaxed family.

It would definitely raise red flags for me.

I’ve read that normalising nudity in the house can be a sign of grooming.

A few things...

1. The nudity and play fighting is completely dependent on the child. If she is still comfortable with it, to me that's preferable over some 13 year olds and their pouting Instagram accounts. I'm surprised she still wants to though.
2. Rudeness is obviously unacceptable
3. Your sister has only been with him 6 months. It's completely inappropriate that she is staying overnight. Far too soon. If anything it's more wrong that shes there, rather than his daughter.