You can't change the trust unless the trust gives you that power. A POA from your mother would likely allow you to do things with respect to her assets not in the trust, assuming the POA includes that power. But it is pretty unlikely to be enough to allow you to change the trust terms, especially who the beneficiaries are and what assets they are to receive. Try to put aside the ill will towards that sibling and just let that sibling get what your parents wanted him/her to have. That would honor your parent's wishes and will save the trust a lot of money should that sibling contest the trust. Also, remember that you have no legal entitlement to inherit your parent's money. They could have decided to give it all to charity and leave you nothing (and that would not necessarily mean they loved you any less). Be grateful that your parents decided to make this gift to you and accept it in the spirit you would any other gift. 

You and your siblings should consult a probate attorney if you are determined to try to cut out your sibling and be prepared for the attorney's fees to start eating away at the assets you are going to inherit. Even if trust beats back the challenge, it is still obligated to pay those attorney fees.