Because I am not a counselor, I can only give you a suggestion based on my experience as a funeral director. It's hard for anyone to lose a spouse, but
to have also lost two sons, is especially difficult. It's nice that you sense that and acknowledge it.
Many funeral homes have someone on staff who follows up with a spouse after the funeral services to answer any questions and to also see how she's doing. If no one has done that, call the funeral home and ask if they can recommend a local counselor who might do that. You might also call her minister or clergyman, express your concerns, and ask if he/she would visit your mother or have someone from the church stop by. If you're not comfortable with that, you might also consider calling her doctor and asking if he/she would speak with your mother or maybe suggest someone who would.
Ministers, doctors, and funeral directors usually have contacts with counselors, grief support groups, and other professionals that they can recommend. Your mother's fortunate to have you looking out for her.