My two sibling cats fighting. Why?

Ok so I have three cats my older cat bud-e has nothing to do with this but my other two cats that are from the same litter have been kind of fighting. One is a boy and the otheris a girl. They are about two years old. I also had a dog. We had to put him down a few weeks ago. Before that Angel,the girl cat, was always distant and stayed downstairs. She always hisssed at jasper,the boy cat, and attached him. But since we put my dog down she has been upstairs more and being with me and my family more. I keep thinking did Angel not like my dog. But she always "groomed" him and layed next to him. Now she isn't so bad but every once in awhile she freaks out on Jsaper for no reason. I wonder why, Can anyone help?