Is it considered a sin to inform the sister of the truth regarding her daughter's cousin's refusal to marry the cousin, which led to the severing of the relationship between the mother and the aunt?

There is no harm in mentioning flaws when seeking advice for marriage, provided it adheres to legitimate Islamic guidelines. If the aunt did not observe these guidelines, she is acting wrongly. The same applies to her two daughters if their objection is without justification. One must verify information. If what was mentioned is proven true, you do not have the right to relay it with the intention of causing corruption, as this leads to sin and severance of kinship ties. Repentance and striving for reconciliation are obligatory. The relationship between Muslims should be one of harmony and affection, especially with relatives. The non-completion of the marriage might be a divine blessing, for "it may be that you dislike a thing and Allah brings through it much good."