Serena vs Venus: My opinion
Serena is better at looking at Venus as an opponent, maybe it's because she's the younger one? You see Serena being angry, she wants every damn point out there, heck looking at her she wouldn't mind going to the other side of the net & punch Venus right in the face, if that's what it would take to win the match. It's as if she's able to completely forget who she's playing and wants to kick Venus' ass. Heck she even said "you fucking bitch" after Venus hit an ace.
However I feel that Venus isn't like this against Serena. Most of the time she seems to have a smile on her lips, it's like she's playing her sister, she can't simply look at Serena and be angry, or 100% intense. I think it's maybe because she's the older sister and she always has protected Serena. She doesn't have the attitude to go all out on her sister.
Anyways as a viewer that's what I notice.