maybe i'm just a contrarian, but i disagree with how you are handling your son's tantrums. what message is he getting? or, as dr.phil would say, "how is THAT working for you?" he is getting your full attention, he is hearing his mom tell him that she loves him, and, maybe more importantly, he is getting one up on his sibling (don't ever discount the power of sibling rivalry).
when my wife and i had two of our granddaughters visit us for the summer (this was over d decade ago!) when they displayed an unacceptable behavior, we told them what they did that was unacceptable and why it was unacceptable. then we had them stand with their nose in a corner for one minute for every year of their age. after that was over, the offending child would have to apologize, stating the unacceptable behavior. then, and only then, did we hug her, give her a peck, and tell her how proud we were of her for changing her behavior.
it worked for us, so i am speaking from my own experience.
i also agree about having a "special" time with your other daughter. focus on her likes and dislikes. taking my middle granddaughter fishing would be a real treat for her. taking the youngest would be a punishment. by the same token, it's always nice to "stretch their horizons," so be as creative as you can.
i hope this helps - at least a little. remember that as they grow older, it only gets more and more complex.