Are you considered to have backbitten your brother when you described to your mother his method of advising his parents as "harsh," knowing that your mother knew you were referring to your brother?

It is not unlikely that what you mentioned to your mother constitutes backbiting against your brother, because it is an insinuation and a criticism of him for not being gentle in his advice. If your brother were to know about it, he would dislike it. The Prophet, peace and blessings be upon him, defined backbiting as "your mentioning about your brother that which he dislikes." So, if you made your mother understand your brother's shortcomings through that speech, then it is backbiting. It would have been more appropriate for you to address your brother directly with the advice, rather than your mother.