I have no easy answers for you. Not every family is interested in their history, especially if there are some complex relationships in the recent past which some might be uncomfortable discussing. For some it’s a door they like to leave locked.
Re the reference to probate documents that Sam has posted for Mary Ann McCluskey, the information tells you that she died intestate (the word administration tells me that). The Probate court in Belfast directed that John McKeown be appointed as executor. Probate was granted on 10th Feb 1913 (ie her estate could then be disposed of as directed by the court, presumably to her next of kin in accordance with intestacy law). Ignore the reseal date. It doesn’t apply in this case.
The papers relating to the administration of her estate should be held in PRONI, Belfast. They will normally say whether she was single, married, widowed etc, and who her next of kin were. The key documents will be a list of her assets and details of who inherited what.