Marth has a few more ways to just create a wall of hitboxes, and Sheik can support from farther using needles. Grabbing someone as Sheik needles to grab break people to their doom is also quite hilarious. If grab becomes the best move to approach with, Marth's throws are faster, so the enemy has less time to try to punish him for using them. At the same, Marth can more readily and easily do faster devastating combos to the enemy on his own, and Sheik's slower throws, and tilts that setup for grabs and fairs can keep the opponent out. While Marth's sword can create a wall and be used to exert pressure, from grounded positions, Sheik can properly punish attempts to escape safe pressure a bit easier. Marth's dtilt is amazing, and generally one of the best normals in Neutral. Sheik has plenty of active anti-airs and strong Keep away moves she can do to cover other avenues, and Marth can commit to dashing forward after hitting with a dtilt without having much worry for the opponent going behind if Sheik stays close.
Marth's utilt can send opponents back to Sheik behind for a pickup.
Super unsafe, but high reward moves like fsmash can be covered for by Sheik from long distances, or closeby.