This may end up a longer post than usual, so please bear with me.
First, I fee you have some of your premises of the question mixed up. In the current constellation, there are not three involved parties in one scenario, but two plus two, in two different problems.
- First, the two siblings in a theoretically equal role.
- Second, you and your younger child, with a (per definition) imbalance of power and responsibility.
But let's start at one end.
In your family, you seem to follow the "no violence, no physical resolution of conflicts" philosophy. Please remember that this is a highly civilized concept, almost philosophical, which requires a great maturity to not only implement ("I am not allowed to hit someone"), but to understand and value ("hitting is wrong, so I don't do it"). To be honest, there are plenty of adults that struggle with this, there are even cultures that follow different moral values. That's a quite ambitious goal, and laudable. Unfortunately, it's only really effective, when all involved parties agree to and abide by this ruleset.
In our family, there's a similar rule, which can (slightly simplified) be summarized as "no hitting". It also includes the rule "no permanent provocation until the other snaps and breaks the no-hittig-rule first". If one child decides that they are going to ignore this rule, the other is also no longer bound by it.
So if your older child gets attacked, it would be ok in my opinion for them to hit back or use whatever physical response they deem helpful at that moment. (I am assuming that you instilled enough responsibility and morals in the older child that they would respond in a way than would cause no permanent damage, e.g. hitting back, but not grab a kitchen knife and go after the younger.) It's a pretty "Stone Age" response, but you may be surprised how effective this approach can be. And there is a reason behind that suggestion: You hint that the younger child acts out in a state when they are the opposite of calm and controlled - in other words, when the logic-driven no-violence philosophy simply won't work because the brain is in another state. Now add to that the facts that the 14yo is in the middle of the brain remodeling of puberty, when self-control and logic actions is difficult plus the stress and the outer limitations of the Corona lockdown. There's a good chance that the 14yo simply can't process moral logic, because they run on mostly the more base brain functions - and the responses must be on a similar level to be understood. So in my very personal opinion, it could help to let some fur fly between the siblings when the 14yo acts out. If the 24yo is physically superior, even better. Let the 14yo find some limitations, boundaries can be a good thing.
That much for the first bullet point, sibling-sibling interaction.
You as a responsible parent should try to stay out of the immediate conflict - unless you were the target of the physical attack. Your job isn't to referee something they can solve amongst themselves. But of course there's a suitable approach: Don't think along the lines of "you did X to your siblings, so I select an arbitrary punishment because I think you need to be punished". A much better approach is "you acted out because..." and then find something that matches the cause. Some examples:
- You felt undervalued.
-> I have a task that will earn you praise, e.g. cooking dinner tonight.
- You were bored.
-> I have a task needing some solid thinking that you should do now. Then you aren't bored any longer. E.g. planning the meals for the next week, checking the available pantry staples, writing the list. Or sorting through their possessions / old books / the attic and selling the no longer needed stuff on Ebay - taking photos, writing the adds, the works.
- You have too much pent-up energy
-> Do something physically demanding to burn it off. (Hint: That could even be a trip to the nearest store to buy groceries. By bike or on foot. Or dropping a care packet at a distant location, e.g. an elderly relative or acquaintance. Of course the lockdown may pose some limitations right now, but this answer should still be usable after the Corona crisis.) By the same principle, the wallpaper in one of our basement rooms gets stripped slowly, an hour or two at a time, music blaring during that time. Chopping wood is also good, or gardening that involves hoeing or digging.
- You tried to piss me off big time (yes, that's maybe a bit of an unfair one, but hey, we parents are also only humans and sometimes need to cement our rank).
-> You get one of the disliked tasks in the family chores rotation. We will still all contribute, but while I unload the dishwasher, you will be scrubbing the bathtub (or the loo).
- You need some alone-time.
-> You get assigned a chore that happens in a room where we aren't at the moment.
- You are aggressive due to circumstances you can't control.
-> I let you "destroy" something. Old cartons that need to be teared down for recycling. Glasses can be smashed into instead of gently put into the container. Or the classic: Weeding. Yes, there my be some unnecessarily forceful stabbing at the weeds, but hey, if it makes you feel better?
In the end, you will still hand out some kind of sanction (someone who's folding laundry can't play computer games), but not for the sake of sanctioning. And who cares whether she's folding laundry and watching TV. So at the core it’s about directing energy from unwanted behavior(= threatening the social balance of the family setup, (self-)destructiveness) to what makes everyone, including the teen, more happy and content.