It can take months for your cats to really settle down.

We have had a couple of kittens for two months and Milo & Treacle still aren't too keen.

I think it is harder to introduce cats when they are inside/outside (Like yours and mine) as the older cats run outside to avoid the new cat.

I suggest giving your original cat loads of extra cuddles and give both cats plenty of time to settle.

All the best with the introductions.