Oh boy, this is outright disastrous. Either the author doesn't understand the concept of soulmates, or they were planning on NTR from the start. There's no special definition of 'soulmate' for A/B/O as far as I've seen, it is what it is in any other story: some, but not all, people have a soulmate, who is very much a SOULmate, a person one is destined to some day meet, a person who is a perfect match for them both sexually AND romantically. The 'signs' of such a bond vary from story to story from an inexplicable subconscious magnetism, to a physical marking, to just Knowing when their eyes meet.
There's stories about rejecting the soulmate system, but in omegaverse, where pheromones and animal instinct trump all, and ESPECIALLY a universe where the soulmate sign is something as overpowering as "instantly going into heat"? Not a chance.
And as bad as Omega-chan looks, the big sister is going to look twice as bad the moment their hands touch, since the farmer's already dealt with this before and there's no reason why it should only affect one soulmate when alphas have their own kind of heat anyway. This is not going under the radar for President-chan, not when her sister and girlfriend look like they're about to rip off each other's clothes in the middle of the street.