One of the most common mistakes a parent can make is assuming you are the boss or have control. Like adults, you will get a lot more co-operation if you explain WHY you think they should change their behaviour. A more gentle way is to play games together where you have to take turns as part of the play. This teaches him about fairness and sharing. Don't expect immediate results but they will come.
Later when it comes to an individual toy I would suggest you get some kiddie safe sand egg timers. (they come in 1,2,5,10 & 15 minutes versions). Start off by taking the toy, getting his attention and explaining that you are going to "take turns", use a short timer (1 Min) and tell him his sister goes first and it will be his turn when the sand runs out.
Afterwards you can use this with any situation, it is also a sneaky tactic to have another toy that each child likes around so that when the sand runs out they can swap toys.
These techniques are often used for Autistic children who lack social skills, I am NOT saying your child is Autistic just explaining the source of the knowledge. If you do not know much about autism visit
http://www.think-differently.org.uk/ or the National Autistic Society main site at
http://www.nas.org.uk/