Serena already hit a 129 mph a few times, but never in, also serena hit a 126mph serve out when playing pennetta. I do think they can serve a lot harder, someone said that at wimbledon, if they optimise their technique and channel more power from the arms, they could hit around 130 mph constantly.
Anyway serena shows by hitting less hard she wins a lot more points than venus (who tries to hit harder and harder each serve) on both serves.
Serena's technique on the motion and a better toss allow her to have better placement no matter the speed.
If Venus had Serena's toss to make her serve disguised, she would make life miserable for even Serena, because Venus has proven (second set against Clijsters when she didn't hit a first serve under 115mph, Sharapova 4R Wimby 2007, Serena Wimby Final last yr) that speed can kill if its consistent and disguised. You'll notice a few players have been able to get reads on Venus' serve and return then effectively before she pulls her toss tighter and in the same spot.
Serena doesn't need speed to win free points because of the disguise, but because Venus has more natural power and can easily ramp into the 120s every match all match (something Serena doesn't have the height to do as much as often), she does it more often.
But Venus' doesn't just crack 120s. She has the best slice out wide serve even over Serena because her height allows her to get it shorter in the box than lil sis, and she can do it in the low 100s where Serena to get it that short needs the speed to drop down into the 90s. So just because Venus' is faster doesn't mean she has any less spin or placement, on the contrary she has excellent production on that particular shot.
The place she seems to struggle the most is the flat down the middle on the both side - she can't seem to consistently get it tight into the center corner - and the short flat out wide on the ad side (which is odd because her height would make you think that would be easy), which Serena can hit on command (ace in the first set TB on set point down vs. Vee in the US Open QF last yr anyone?)
That being said, Venus has the record with Brenda Schultz McCarthy at 130, and she has hit 131 (wimbledon 2004 2r) and 133 (AO QF vs. Hantuchova) out before. So we know it's only a matter of time before racquet technology will move that speed up.
I mean come on, do you think Serena is any stronger now that in 2002-2004 when she topped out at 124-126? Not at all. Venus also never matched that 127 speed until 2007, and especially since her abdominal injury she would always top out around 123-125 in a tourney and not hit them consistently. Venus is also no where near as strong as she was in 1998-2000 when she was a beast like Serena. She is a thinner less muscular person now with better technology. Racquet technology allowed them to move speeds up. Hence why BSM was able to hit a 130 at such an age, not because she was more powerful at 30+, but technology allowed it. It doesn't have to be drastic, just slight advancements in frame and string technology. Even stringing racquets looser would make it possible.