sibling rivalry

As a mother of 4, you'd expect that I know how do deal with sibling rivalry. You'd be wrong. You see, my oldest was 4 when my second arrived. He adored his brother. Then when my second was found to be sickly, and then became disabled, my oldest made up games that allowed his brother to be a part of it without being able to physically play along. Then, when I married my husband and inherited my sweet step son he and my oldest were already great friends. And also old enough to talk through any issues that arose between them. 

When my third biological child arrived my second was 3 1/2 and completely in love. But he also passed from complications with his diagnosis 6 weeks after his baby brother was born. Then along came the baby. My 4th biological child. He was a joy to us all. And his oldest two brothers dote on him. My now 3 1/2 year old, 2 1/2 at the time, seemed to as well. 

BUT NOW.... 

Now my 3 year old is MEAN to his 1 year old brother. Steals his toys, pushes him down, and is just not nice to him. And the youngest, UGGGG! He pushes and shoves his brother and has taken to screaming when the 3 year old tries to hug him. And it's not like I can REALLY blame him! I'd scream too if someone who is usually mean and even physical with me came near again! I'm just so tired of the yelling. The baby is teething, again, and is FINALLY playing quietly for the first time in hours. So I begin dinner. Then not 5 minutes into this blessed semi silence, my 3 year old comes in and tries to steal the cars from his brother. Cars he doesn't normally even want to play with anymore. He's now taking the extreme pleasure in getting very close to his brother until he starts crying "No!" In a panic. How do I fix this?!? No matter how many times I've tried to sit him down and explain that his behaviors hurt people, and hurt his relationship with his baby brother, he does it minutes later. 

My only conclusion is that they're simply too close in age, yet not close enough. Pray for me. I certainly could use it right now.