TBH this sounds like perfectly normal behaviour that happens between brothers and sisters.
So how about seeing it as a compliment to your blended family - the 8 year old is behaving like a NORMAL brother. Well done to you.
So treat him as though he was one of your own - sit him down and tell him that teasing a 2 year old is very unwise. That it will (as he has discovered) more than likely lead to a massive tantrum - and everyone being cross.
Ask him if he knows why he does it (he prob won't - 8 year old boys don't really have much upstairs ).
Think about why he is doing it. He's older now and his understanding of family dynamics and hierachies (like who is the "most" important child)is growing - he knows he's not "yours" - he knows he's an interloper in the family (however welcoming you are).
Is there a chance he is testing out your "evenhandedness" or "fairness"? Something along the lines of "does she really care about me - am I less important that these two little siblings? Who will she tell off if we fight?"
IMO you have to sit down and have the chat about how he feels and point out how foolish he is being. He can understand that at his age. And you can tell him what the sanctions will be if you see him taking things from your dcs - unless of course he had them first and they grabbed them from him!
And finally tell him how much you value him as part of your family - spell it out - you are tired, your dcs are difficult and tantrum-y and you like the fact that he's grown up and responsible.
And get his dad to do something "boy-sie" with him every weekend - play football, go to the cinema, play computer games or board games. You stay put with the little ones and dad should take his son out?
No 8 year old wants to be stuck with little siblings all weekend!