It shouldn't really benefit her too much. Sheik is harder for Pit to camp than Zelda is despite her reflector and projectile, so its entirely possible for Pit to end up building up a lot more damage against Sheik + Zelda assuming the player switches to Zelda to get the kill.
If Pit decides to just run away and camp to gain percentage then, Sheik's speed would probably be more advantageous in comparison to Zelda's power as far as getting a KO is concerned.
If Pit is going to run away, I want to know how he's going to KO Zelda. I also want to know how Sheik's speed is such an advantage here while Zelda's power is not. We're not fighting you on Bridge of Eldin. Where is Pit going to run to so Zelda can't catch him? He glides quickly, but his running speed and his aerial mobility aren't blindingly fast. Running away only helps when he has a lead, but really... where is he running off to? Maybe you meant he's playing defensive, which is different to me than running away.
You pull an arrow out, and I'll 1) throw Din's at you (or the ground in case I get hit so it'll still detonate), 2) wait a bit and Nayru's, because the reflector lasts a long time and it isn't impossible to react to when the arrows will come out, or 3) powershield the stupid thing. Once I'm close to you with Zelda, I don't see why Pit should still be doing better than her. She generally has more range, more killing power (and no, don't tell me that being lighter than Pit means Pit has more killing power, because Zelda is killing pretty much everyone she faces soon, and her most potent killers that don't involve meteors happen to be the strongest aerials in the game), and, taking Pit's range into consideration, better speed with her attacks.
I know Pit has some quick moves up close, but with dtilt, Dsmash, and Fsmash alone, I can pretty much beat out every other ground move he has. If you decide to start taking to the air against me, well, Usmash, ftilt, and utilt will be there to answer, with more range (enough to hit Pit before he hits her or a trade), a lot more power, and definitely more speed. What significant advantage does Pit have here?
Your aerials are good, but honestly, people (maybe not you guys in general) think that Zelda is going to dog fight you in the air, and they also think that because they can't consistently sweetspot with her in the air, she'll probably never get you. Her aerials are not good for approaching, and... that's about it. Punishment out of shield? Yes, and we know how hard they kill. Forcing an airdodge/other predicted reaction? Yes, because nobody wants to get hit by them. Killing you? Ooooh you betcha. Of all of this, the OoS responses and the killing power are the main things we're looking into for this aspect, since making a mistake is a player's fault, not a character's.
When it comes to edgeguarding, I'd give Pit a slight advantage here. Zelda's recovery is not the best. Pit should wait on the stage and fire arrows to harrass her and then punish when she warps. Going off stage can lose your advantage since she can warp past Pit. Zelda can stop your glide with ease, so just don't glide and you should be ok.
Overall, this fight is pretty much even. CPs will be what will sway this fight.
Also, if Sheik vs. Pit is even, then Sheik/Zelda vs. Pit is easily their advantage. Not only do they cover each other's weaknesses, but they can also CP per stage. I cannot think of a single stage that's bad for Zelda, bad for Sheik, and possibly good for Pit.
I'm not trying to start trouble or anything, but seriously, I looked at the match-ups you guys have listed on the first page and could not stay silent.