I need to do something.

It sounds as though she may be moving on, in which case your problems will be greatly reduced, but the main problem is your father. Chasing away all the bitches he ever dates is OK but they're attracted to him like flies on shit. If he wasn't such a pussy then the sewage wouldn't always be washing up on your shores. Firstly, from what I can tell, you're the greatest asset that company has. It's just a shame you don't realise it and don't use that fact as leverage. He'd be lost without you. You're the perfect employee in that you work your ass off and take any amount of shit from this Cheryl at the same time. Actually, not so much her shit as that from your dad. I know you're incredibly loyal to your family and your father but when are you going to realise that just because you're related it doesn't give him the right to fuck you over? It's not your duty to help him and his company come hell or high water.

Your skills and work-ethic could get you hired in a considerable number of places, and with people who won't treat you like the office doormat just because they're "family" either. Then it'll just be the rules of the workplace without someone close to you being able to bend them and expecting you to tolerate it. Misplaced loyalty and pride and bottling up your rage hasn't fixed your problems thus far. I really think for your own good that you have to leave that place. That'll make them suffer more than anything, without the need for childish pranks or anything else. I don't mean that you should completely ruin your dad's business but he sure as hell needs one almighty kick up the ass. Cheryl isn't really the problem; if it wasn't her, it'd be some dumb bitch by another name ad infinitum.

And don't think that leaving would be backing down or admitting defeat, that you couldn't drive her away or something. It's just being smart, because it'll make your life a lot less stressful and teach them a lesson to boot.

Jam it back in, in the dark.