Depends on if the Marth knows what to do vs Sheik (which is usually not often).
If you play a campy, waiting Marth you will probably be able to run all over him through aggression. You could even make him approach with needles, but chances are he won't know where to put his sword to keep Sheik out since he will still remain relatively defensive.
Unfortunately, Sheik has very limited options vs a Marth who is safely aggro'ng you. Marth's dtilt destroys Sheik's entire ground game and will be used to bait rolls, don't fall into it. When you're being zoned the best option is to probably dash once backwards to reset the situation and reevaluate from there.
Marth can't get out of two ftilts > follow up, so just rely on that and don't get greedy with more tilts. Ftilt > up smash will kill relatively early if you are able to degenerate it throughout the stock, but this is one of the match ups where it will be pretty hard for Sheik. If you ever get Marth off stage try your best to be able to capitalize, she definitely has the weapons to gimp Marth. Also grab a lot if you can and get Marth above you where he has very limited options.
If the Marth has experience in the match up it's still 60 40, it all comes down to his zoning which makes it tough for Sheik. Do your best to try and not get zoned.