Does signing a trust receipt for a large sum of money without receiving the actual amount constitute a breach of trust?

You erred by signing the receipt unjustly, and this is not a way to resolve marital disputes. The correct approach is for two arbitrators to be sent to reconcile between you. What your husband's sister did by exploiting the receipt to demand money from you is an injustice, an aggression, and an unlawful consumption of others' property, which is forbidden and considered among the major sins. Religious people should intervene to advise your husband's family and resolve the dispute. We advise you to return to Allah, make supplication, and reach an understanding with your husband.