It's a little unclear from your question whether the services have already taken place. The person who signed the billing statement is responsible for its payment. The funeral home is
required by law to fill out an itemization of the merchandise and services that are chosen and their prices. The person who is responsible for these selections and their payment is then required to sign it and the funeral director is required to give a copy to that person.
If the services have not already taken place, call the funeral director as soon as possible and express your concern over your sibling's ability to pay if he signed the agreement. The funeral director will then talk to your sibling about changing some of the arrangements. If he was not the one who signed it, whoever did sign should call to change the arrangements to fit what he/she can pay.
You may also want to check on any resources your mother left that can be used toward payment.