Is one who, with the ability to perform an emphasized Sunnah—such as the prayer for rain (Salat Al-Istisqa)—and without a valid excuse, fails to do so, considered sinful?

The Rain Prayer (Salat al-Istisqa) is a confirmed Sunnah. One who omits it does not incur sin, because the request to perform it is not in the nature of obligation or compulsion. However, persistently abandoning it is considered a deficiency in one's religious practice and a breach of chivalry (muru'a) by some scholars.