50:50.
Fortress is very important in this matchup. It's his fastest attack with frame 1-5 invincibility and considering Sheik is statistically the fastest attacker (yes, faster then MK). Shield and Fortress, shield and fortress. F Tilt lock? Unless he has the appropriate decay and percentage (in which case, yes you're stuck until you SDI out and Sheik finishes it with U Tilt, N Air or U Smash) then you fortress that ****. This can apply to virtually any of Sheik's pressure and pseudo combos, you must live in that shield and respond OOS.
Sheik has a broke *** chain (a lot of characters simply cant get through it if used properly). Luckily, firebreath can outrange chain and also, between each hit of the chain there is a 2 frame gap, so if you do get caught in it in you can Fortress spam it out.
As usual, you space on the ground with F Tilt and Jabs, your priority is keeping Sheik out with your marginally superior range. Also, you don't take to the air unless necessary, Bowser will be beaten in the air by Sheik.
Bowser has to approach, Sheik's needles are a good projectile, whether they'll charge for a full needle storm (18%) or if they'll spam single needles, both are effective and means you've got to get in close proximity whilst still spacing yourself out of her close combat game. That's the zone you want.
Sheik's an effective gimper but if she's coming for you, time that fortress and abuse the invincibility and priority. Sheik also cant chainguard well because of this. In regards to you edgeguarding Sheik, edgehog rules Sheiks recovery. Chain recovery is fast and effective and you take that option away with an edgehog, you instead force Sheik to use the much laggier Vanish, so predict her landing from there and punish accordingly.
Sheik has a marginal problem with killing and Bowser is super fat so you can expect to be living much longer then Sheik. Sheik's ways of killing are gimping (covered), D Smash/B Air and U Smash. F Tilt decayed properly can combo into U Smash which IIRC kills Bowser with DI at ~115% on FD. So your job is to try and keep that F Tilt at inappropriate decay, either by avoiding the F Tilt all together or fortressing when it doesn't combo.
For the record, Zelda used appropriately will always be a boon in this matchup, both with killing and general play.