A: In New York, the names on the bank account trump the will, or more accurately, the bank account passes outside the will. That your sister is executor of your mother's estate is irrelevant. If she is the only one on the bank account, she gets the account. If she is a co-beneficiary on the account, she shares it with the other named beneficiary or beneficiaries.
A: The attorney may be concerned about privilege, since the lawyer-client privilege might not be effective where a third party (you) was involved in discussions. That aside, since your husband is a trustee, he can choose his own lawyer (who may or not be joint with his sister's lawyer). In fact, I am not sure what the mother's lawyer's position in this whole thing is.