Marth's fair beats sheik's fair. Marth's fair also beats sheik's bair but you have to space anticipating the leg sticking out. Sheik actually really doesn't edgeguard Marth that well if you take the right angle, and not at all if she doesn't have time to get on the ledge. Plenty of the time she has time for this, but not always, and she's still just going to force you on stage unless you make a mistake and get bair'd. Once she gets you on stage, she'll stand up and grab again or ledgehop dair and try to continue combos in towards the stage which you will eventually at least escape with a chance to do something back, or at certain percents she may be able to just kill you with up air/up smash.
As bad as that may be, it doesn't take a whole paragraph to describe what happens to falcon offstage vs Sheik. Falcon just dies.
At 0 Marth can CC grab a lot of sheiks stuff which is great because you need to grab her. He also has very small windows to pull off big combos on sheik that will often require a well-timed, well-executed edgeguard in order to get a kill and avoid the nightmare of dealing with a sheik at too high % to combo (like 75%, lol) but too low to kill for a nice long while which is clearly the point at which Marth loses (needing many openings vs an opponent that consistently, at any %, gets a whole lot from one opening).
Compared to Falcon, Marth is much better equipped from neutral stance and also in recovery, while CF has better punish on sheik. They're both < Sheik imo.