Can my husband help his brother with mother's finances if he is not the POA, legally? Brother is POA.

As already asked, the answer may turn on what "help" is involved?    Consultation, advice, or hands on management?  

Could you elaborate on what's under consideration, and the relationship between the two brothers?

It depends what sort of help you mean.

Keeping accounts, making and recording purchases from petty cash or on a dedicated credit card, doing the work, yes. Making key decisions, entering into contracts on your MIL's behalf, representing her in financial matters, no.

Essentially, I suppose - anything that would require his mother's signature or consent if she were still acting for herself must be handled by her POA, and those responsibilities cannot be delegated to another person.

I don't know what you mean by "help with finances".

I believe he can advise and offer opinions, however, he cannot sign anything and any decisions formally made have to be made and presented by your BIL. Your husband cannot represent his brother in any situation. Anything that needs to be signed or done as POA. must be done by your BIL.