Really sad that we lost all the archived posts from the old forums - actually had a few write-ups on this topic I’ll do this in two parts, this first post to kind of address the general game plan, and then one with some replays that I can kind of analyze.
Generally speaking, your goal for this fight is mostly to control space. Your job as Hisako is to keep your opponent from getting in that space right above you. You either want to be fighting Sadira from point blank range (on offense), or from roughly mid-screen. This is the only fight where I encourage Hisako to not try to be full on wrath all the time - your options are much better served by throwing out long normals and trying to punish Sadira’s approach. Hisako actually controls a lot of the same space that Glacius occupies when he fights Sadira, that box directly in front of her that catches anyone trying to contest her in the air. Jump+HP is your best friend in this fight, as it will beat anything Sadira tries to do in the air on approach, and none that I’ve fought are conditioned to try to catch limbs the way they do with Glacius. Use jump+HP to bat Sadie out of the sky when she first jumps, and don’t worry too much about whiffs since she can’t really punish it.
The other big thing in this MU is aerial ORZ. Jump+HP is good to knock her down at extreme range, but aerial ORZ is what you’ll use to truly punish her for approaching incorrectly. Any medium widow’s bite done in mid-range (where the kunai does not actually hit you) should elicit an air-ORZ on reaction. Medium widow’s bite is slow and bounces Sadira up in a very predictable trajectory - it is virtually always punishable if the kunai isn’t locking you down. You may need to alter the strength of the ORZ to catch her depending on the range, and you may need to use jump+HP in some situations, but in general any medium kunai in the midrange can and should be punished. If they’re wise to this and use heavy or light widow’s bites, or are just jumping and double jumping without taking any other action, then revert back to jump+hp to check them - it’s fast enough and far enough to do so consistently.
Once you’ve knocked Sadira down, treat it like every other MU with Sadira, where your goal is to ground her and then keep her there. Hisako’s pressure works just fine on Sadie, with @Marbledecker’s caveat that cr+MK is not the ideal meaty button to use against her. I personally use the hard knockdown ender more on Sadira than many other characters, simply because it guarantees the wrath refill that’ll let me counter-cancel or recap off any landed air-ORZ. Whatever your preferred pressure option, however, your goal is to keep Sadira grounded. Once she gets instinct, use a setup that’ll catch an activation web and keep her pinned down.
All of the above is on how to play neutral and offense, but obviously that’s not the situation you’re always going to be in. Your goal for the entire fight is keep Sadira from being right on top of you, but it can and will happen, particularly against a good Sadira. Hisako has decent options to deal with Sadira’s pressure, but how well those work depends entirely on whether and how quickly you are able to get a feel for how that Sadira likes to pressure. My personal experience has been that Sadira has to respect counter as a hard, well, counter to her mix-up game, and so the preferred offensive option will be to come in empty and then throw or cr+lk you, or use light widow’s bite into the same. Be mindful of empty jumps off kunai and respond accordingly. A single throw tech will reset your spacing such that you should be able to get out and back into more favorable neutral. In these cases I recommend jump-back+hp - Sadira will tend to follow throw breaks with a return to the air. If you can manage it, an influence command grab on her landing frames in these situations is the optimum punish for coming in empty off a widow’s bite. On the flip side, know that on occasion she will try you and will come in with buttons, so be mindful of that and do still have your counter option ready. Understand that for truly optimized Sadira/Hisako play, the Sadira has to be willing to eat a counter or two - that’s how she establishes that she is willing to press buttons and make you whiff counter later on.
Once Sadira gets instinct - get out. You need to escape Sadie once she’s in instinct, as she is more than capable of wrecking Hisako once she establishes an uncounterable, safe left/right/throw mixup game. If you manage to get out, immediately up+back away from her and keep running until her instinct ends or she does something off a jump that’s punishable. If you’re running out of stage, Hisako’s command teleport is a good way to get under a jumping Sadira and recommence fleeing in terror.
And that’s pretty much my (not so) short take on the Hisako v Sadira MU. I think the fight is a solid 5-5, as both characters are quite capable of destroying the other if they’re not on their game. As @Tswagg said, Sadira can do quite fine in this MU - your job as Hisako is to control the neutral such that she doesn’t get that opportunity. I’ll post a set of replays after I get home to discuss the MU a bit more.