Sarna, an orthodox jew, marries nathan, a reform jew. sarna is disowned by her parents because, as far as they are concerned, she has married outside their religion. she has violated the rule of:

endogamy

Explanation:

Endogamy -

It is the method by which the people get married within the similar social groups based on caste  is known as endogamy .

This type of practice is very commonly seen in many cultures and religious groups , where people tends to get married within their community , religion and caste .

Since , Sarna married Nathan who is from a different religion , and hence , she has violated the rule of endogamy .