Need help getting my cats to get along

This will be my first post in this community, but I hope you all will still take me seriously. For some context, I have two cats. Marigold, who's 11 years old, and Eevee, who's around a year old. I've had Marigold ever since I was little, and he's a very jumpy cat. Unfortunately, he's always been an outdoor cat no matter what my family has tried, which I heavily dislike, but that's not the point. The point is, Eevee is a new family member, and she's extremely skittish when it comes to other animals and tends to be very aggressive toward them. We think it's because she was in a bad home environment before ours.

I've genuinely tried everything to get them to coexist, but my grandparents, on the other hand, let them meet face-to-face the day Eevee came to our house without my knowledge, which was obviously a huge mistake because that's not how you should introduce cats. Since then, Eevee has begun chasing Marigold out of our house and scaring him through the windows. I've tried keeping them in separate rooms with treats, and I've also tried brushing one cat while the other sniffs it. I'm just at a complete loss here.

My grandfather wants to kick Eevee out of the house because of her attitude toward our other cat, and I cry every time he yells at her. I just really need advice on how to get them to at least tolerate each other. Eevee is my mood stabilizer, but I love Marigold with all my heart. Any advice and help would be greatly appreciated; I just don't know what else to do.