My brother has POA and doing mothers bills. Can he legally use her money to buy a new computer just to do her bills?

Why does he need conservatorship if he's got her Power of Attorney? I smell a rat and I think you do too. Listen to your intuition! Call your local area agency on aging or speak with a family law attorney in the state where your mom resides. Ask them what the legal and ethical responsibilities are regarding Power of Attorney. As her PoA, he's agreed to look out for her best interest. If her bills go unpaid, if her needs are not met, if her physical environment and cleaniless are overlooked while her funds are spent on things OTHER than her care and tending- HE might be held responsible. "conservatorship" in my state is like "guardianship" which means he'd be on the hook for her bills if she can't pay - even if she's out of $$$, brother would be liable. So - clearly understand what the state law considers elder abuse and make sure brother understands it very clearly. He might rethink his spending and his desire for "conservatorship". Good luck to you and to your mom.