When your husband petitions for his mum, she will be considered an Immediate relative of a US citizen, this status does not allow beneficiaries of the I-130 petition, so your brother will not gain anything through this particular petition for your mother in law, that is why the CSPA will not apply.
CSPA would not apply. A separate petition is required for the mother and the brother. The brother faces a wait of over 10 years before being able to immigrate.
J Charles Ferrari Eng & Nishimura 213.622.2255 The statement above is general in nature and does not constitute legal advice, as not all the facts are known. You should retain an attorney to review all the facts specific to your case in order to receive advise specific to your case. The statement above does not create an attorney/client relationship. Answers on Avvo can only be general ones, as specific answers would require knowledge of all the facts. As such, they may or may not apply to the question.
The CSPA would not matter because his mother, as a parent of a US Citizen, is an immediate relative and, as such, her son would not be able to "piggyback" on her petition. He would need to immigrate on his own petition, which would be a sibling of a USC and the waiting times for that category are over 10 years under the current laws. Good Luck.