Kate has a good point with using a "time in" rather than a time out. He might do better with time in with you rather than being sent off to the corner etc. by himself. You might try sitting on the floor with him between your legs or sitting on the couch holding him. If he fights too much, you might make the rule that he has to sit next to you but not on you. Point is that he has to be with you and not away from you. In his mind, he might be thinking, "if I do this, then she'll push me away and I don't have to get close to her. She doesn't love me so I don't have to love her". A time in can start showing him that even when he does the wrong thing, you are not going to push him away.
You might also find time to do more attachment and bonding activities with him. Like holding him while he drinks out of his sippy cup, rocking him, laying with him before bedtime and forcing the eye contact, even if it's just for a minute.
And just me personally, I would make sure that you meet all of his needs right now. Do not let his older sister get him anything, even a toy. Or help him get dressed etc. This will help both of the children...let her be the child she needs to be and help him learn that it is a parent's job to take care of him. Right now he doesn't know that because his parents didn't do it for him.
Hang in there!
