I have three adult male cats and none of them like each other, but they all love us and have (separately) great characters. I have learned over the years to leave them to sort it out for themselves, apart from giving them a good shouting when one has stuffed the other one under the sofa and they are are both yowling and moaning. They are all of a similar age and all arrived at different times so setting up the pecking order has been difficult. The important thing for your cats is that you spend equal amounts of time with each of them so each feel loved by the humans, you are top cat to them. Black one of mine loves my OH so listens to him, the others defer to me. We are fortunate to have a tall house and over time each of them has found separate sleeping areas far enough apart not to bother each other.
I had a female cat which died young and the tabbies never got on after that - it seems that changes in the cat family (like when your old cat died) take a long while to settle, she sounds unnerved by it. She probably feels the younger cat is a threat at the moment, but they will eventually sort out some sort of uneasy peace between them. I don't want to put you off, but mine have taken five years to reach the state of truce they're in at the moment! Good luck with it.