OP will not have a problem but when she tries to sponsor mom and brother later, it will become a question if brother will be too much of a strain on Canada's system.
The brother at age 28 is only considered mom's dependant and able to go in on her application if he is financially dependent on mom since before age 22 because of his disability. If he is, then it becomes a question if immigration will let him in anyway since he is unable to support himself and OP as the sponsor is only required to support him for the next 10 years. I don't know if CIC denies people for that reason or only for medical reasons. OP's brother is healthy and doesn't need medical care but he does need money to survive unless he can work.
Then again, if he is so well off that he can work and take care of himself, then he is not considered mom's dependent and can not be sponsored by mom or OP except possibly under a family stream of a PNP in certain provinces but then he would have to fill other requirements.
Best idea I have is go ahead and apply for PR and when it comes time to apply for sponsorship, see an immigration lawyer about how it would be best to pull this off.
edit: You can not include your mom or brother on your initial application. You can only sponsor mom after you become PR and it takes a long time. Brother can come with mom if he is considered a dependent of hers, OP can show enough income to provide for both of them and CIC doesn't deny him for medical reasons.