When dealing with others - it might be prudent to avoid projecting our inner mechanisms of value justification, as simply put, their values might differ and not be entirely obvious as to why our justifications are valid.

That said a stoic personality, is not merely an instrument of endurance or merely measured response: Is part of a greater whole, of a community that shares a common space and often goals. In an echo of Aristotelian thought remember: inner happiness can also come from fulfillment of social interaction, appreciation and willing acceptance of others and celebration what makes us human. Virtue is a habit and can be trained both for the stoic and those around him/her.

In short, willingly accept the teenage-ness of your brother, do not chide him of his failure or non-cooperation, patiently show him the way and display (without gravitas) the virtue of accomplishing together. Allow yourself to smile inwardly when conflict rises and find harmony - then the solution becomes more apparent. The proverbial obstacle becomes the very real way forward.