ANSWER:

The inheritance must be divided according to Sharia law.

If signing this document would allow your mother to distribute it accordingly, there is no problem in this to avoid the tax though I had encountered tens of similar cases where the mother seized all wealth for herself and refused to give her children a penny. I personally would not agree to such a thing and would rather pay the taxes rather than losing the whole thing. You should know your mother better than anyone else. 

QUESTION: My father passed away and left no will My mother is still alive and my siblings are trying to avoid inheritance tax, by passing down the inheritance to my mother. Is it allowed in Islam for me to agree/sign so that inheritance goes all to my mother