My question involves estate planning in the state of: New York

Approximately a dozen years before my mother’s death her long-term lawyer (40+ years) created for her an irrevocable trust in which her second, vacation home was placed. The trustees were my sister and myself. 10 years later, approximately two years prior to my mother’s passing, it was discovered (by the lawyer) that the trust was never properly executed and, therefore, never existed.

When confronted with the fact that over the years the bogus trust cost my mother over $50k in lost tax advantages (among other things)--some but not all recoupable by filing amended tax returns, my mother’s lawyer and one of the law firm’s founders said that in fact it was to my mother’s benefit to now be freed from the irrevocable trust, so no big deal---oops!. After my mother’s recent death, the lawyer hired to probate her will said that there might be a substantial (20%) New York state estate tax hit on her estate because of the value of my mother’s second home.

What, if any, recourse—legal or otherwise-- do I have in New York to force the lawyer and/or his firm make my mother’s estate whole?