The most effective time-outs with my son at that age were when I removed all interaction between him and myself--that way he didn't get any attention from me for his negative behavior. Here are a few ways it looked at my house:
--I had a child gate at the kitchen entrance, so I would close it between us (didn't matter safety-wise which side he was on), and I refused to look at him or talk to him for two minutes (or one minute when he was one). He would inevitably stand at the gate crying and trying to interact somehow with me.
--Or, since my bathroom was a also child-safe place, one of us would go into the bathroom during the time-out.
--Or, if it happened in the car or store (he was strapped into his carseat or shopping cart), I would tell him he was on a time-out and refuse to look at or talk to him even though he was right there.
The two minutes would seem like an eternity because he would cry and carry on and shout for me to look at him, but I stood firm and watched the second hand go by. As soon as two minutes were finished, I would look at him again and explain again why the behavior was unacceptable, remind him not to repeat it, have him say he was sorry, and give him a hug.
I'm not sure how your house is set up, but if there are no safety issues in your son's room, and the things he can get at to throw won't really hurt anything (you could put those on a higher shelf), then you could try putting him there--BUT ignore the tantrum and throwing. It gives him attention for the negative behavior, and that will actually reinforce it instead of eliminating it.
So that was the key to time-outs for me--removal of interaction, rather than a specific place that he had to sit.
One more thing, about sharing toys with his baby sister....It will be good if you can give him words to say when he wants the toy. We do a lot of "scripting" at our house (mine are 21 months apart). I would make my son say "let's trade", and he would have to trade to get what he wanted from his baby sister, and then I would answer for sister "ok, here you go." When they were a little older it was "may I have a turn in 1 minute?", and the timer would be turned on, and when it rang I made my daughter give up the toy and said for her "it's your turn now, can I have a turn again in one minute?" We've gotten more advanced than that now that they are 5 and 7, but it made my son feel like he had words to say and that his baby sister wasn't always at the advantage with having whatever she wanted.
Best wishes. Hope you find some strategies that work for you.