Can a Sinner Be Effective in Prayer?

I just got word that a friend’s cousin had a stroke. She is healthy. Thirty-nine. The mother of three. She had a hysterectomy a couple years back and they say it was the hormone therapy that triggered it. I pray for her. But do my prayers count? I pray daily, with empty promises as I've admitted in my recent posts. When I couple prayers for others with those, do they mean anything?