My oldest cat is marking the carpet/ attacking my recent addition and I need advice

Okay for context I have 3 cats. My oldest is Omelette she’s a Torbie (tortoise shell tabby) and I have 2 boys. Parfait we adopted as a kitten which Omelette pretty much immediately accepted. Then there’s Mimosa. Mimosa was a rescue from the dumpster at work. He was already about 8 months old. Omelette and Mimosa tolerated each other pretty okay for about 2 months with little spats here and there. In the past 2 weeks Omelette has been very aggressive with him. She’s chasing him around the house and swatting and growling at him while he actively tries to get away. They’ve both marked in front of the large cat tree we have in our living room. We have 3 litter boxes and I plan on adding a 4th I just don’t have a ton of spaces to put litter boxes. Omelette has a history with territorial marking and yes she has been to the vet to rule out any medical issues. She’s just like that. She seems so stressed out when he’s around and I haven’t seen him be overly aggressive with her so idk what the deal is or how to fix the situation. Her and Parfait are totally fine and still affectionate with each other. She’s always been a loner but it’s never been this bad.