You need to nip this in the bud. Everything has a value, and this value has to be split 4 ways.
Anything that has an agreed Nill value by the 4 of you, is either to be given to a charity or discarded.
The elder brother is already showing complete disregard for his position and your mothers wishes, and he need to be aware you will not be made a mug of.
And finally, I don't see a happy ending with your siblings no matter what actions you are to take, as they already have shown a complete disrespect for you and your mothers wish.
Take them on, and good luck.
Ls