Might it be that sister? Poor nun in trouble!

Wiktionary Word definitions in Wiktionary
n. (context grammar English) A pronoun that introduces a relative clause and refers to an antecedent. Some words that can be used as interrogative pronouns can alternatively be used as relative pronouns: ''what'', ''which'', ''who'', ''whom'', and ''whose''. ...

WordNet Word definitions in WordNet
n. a pronoun (as `that' or `which' or `who') that introduces a relative clause referring to some antecedent

Longman Dictionary of Contemporary English Word definitions in Longman Dictionary of Contemporary English
noun EXAMPLES FROM CORPUS ▪ The most notable is the relative pronoun that, which can only be used with a restrictive relative clause.

Wikipedia Word definitions in Wikipedia
A relative pronoun marks a relative clause ; it has the same referent in the main clause of a sentence that the relative modifies. An example is the English word which in the sentence "This is the house which Jack built." Here the relative pronoun which ...