As long as antohass has no intention of sponsoring his mother for PGP, his PR status is safe. Once he submit his PGP request, IRCC will notice that antohass has omitted his brother from the family information form, in his own original PR application since the application will asked for all siblings, unless he omits the brother from the PGP application once again. In other words, any future applications with IRCC will have to pretend his brother doesn't exist.