What is "talaq al-tunazzuh" (divorce for the sake of integrity/abstinence), and is it obligatory or recommended if a husband discovers, two years after marriage, that his wife is his sister through rada'a (nursing)?

Lactation prohibits what birth prohibits. If the suckling is doubtful, the woman is not prohibited on account of it. However, it is recommended to abstain and separate from her. This falls under piety and caution in religious matters, in accordance with the Prophet’s (peace be upon him) saying: “Leave that which makes you doubt for that which does not make you doubt.” This caution is recommended, not obligatory.