I don't doubt or argue biden is a bit creepy...or that sessions seems to keep the distance between his grand daughter and biden, but here's what I feel a more clear take on what is occurring in the video..true, Sessions brushes his hand away and true Biden says "what happened" but it if you watch closely, what happens is her hair gets caught and sessions says "Oh,Hello" at 7 seconds of the https://youtu.be/Hdms5kl5BMMYouTube link in the OP. He's messing with her hair at about 10 when the zoom to biden, and you hear sessions say "it got caught" - referring to his grand daughter's hair and it looks like biden reaches to help at the tail end and it sounds like sessions says "don't touch, she's a tough girl" and that's when the swat happens and Biden says what happened over the top of sessions comments.
It definitely it appears that sessions has reservations about Biden being close to her for sure. but it also appears that Biden reached over to possibly help. Sessions doesn't seem to want to be the buffer between the two certainly though.

In the non-zoomed video https://youtu.be/EoLHn577xIE you see it clearly as mentioned by others around the 21:35 mark.