I adopted 2 kittens and have had them for about 5 months. They have always gotten along great until yesterday. I brought home 2 new kittens and one of the older cats has been fighting all 3! I'm so lost on what to do! It would make me so sad to get rid of any of them, but I cannot stand seeing the original 2 not getting along! I cried about it for a few hours yesterday and just need help/advice! Is it possible to get them to get along.

The original cats are sisters and the two new cats are brother/sister.