You are at a hard age for cooperative play with both of them but it will get better I have some rules that I implemented since mine were little and it has developed a great sibling relationship.
So this is the deal in my house....
1) If a toy is used to hurt another person it gets taken away(It could be the rest of the day, or just a few minutes butit is gone no matter who did what) and explained that it has to go if it is being used that way. In the same way if a toy is fought over and it comes to blows...same thing.
2) If said toy is used a few times it is explained that it will be given away...gone... if it is used that way again. And then it goes if it is used that way again. Freecycle.com
3) The words are..."when you're done can I have a turn" and the response is "whenI'm done you can have a turn" That interaction must me respected....with no favoritism. I often help the other one find something equally as fun to play with "while they are waiting". Yours is very young for this but my 19 mo old says "turn" as her part of the conversation and that is fine, she know whats going on.
4) The oldest (4 in may house) has to know that the little ones are little. His "job" is to teach them the words and often teach them how to use toys/games etc. so that he has control and feels like a big boy and the little ones begin to trust and woork cooperatively with the older ones. This relationship should be fostered all the time. Often I ask my older on eto hold the younger's hand while crossing the street (as I walk close by, but he gets the responsibily of teh job) HE LOVES THIS! He shows her candyland, puzzles, trucks...sometimes frustrated but she wants to watch him and do the big kid things so is tryingto work cooperatively.
On the same track, they LOVE doing mommy things, breaking up broccoli into pieces, mixing pancakes, cleaning the table. This can be used to your advantage. I said last night...if you don't clean up the puzzles you can't help me "cut the mushrooms for dinner" and the scurried around to clean up!
5) Hitting is never ok. Help them use their words to say I DON'T LIKE THAT, Don't hit me etc. so they get a chance to tell the other how they want to be treated but then the resolution MUST come next. At their age, that's your thankless job to mediate. Books helped by son not bite "teeth are not for biting" and there is "hands ar not for hitting".