My sister asked me to watch her baby, and I fell asleep

Dear Delyara, My sister asked me to watch her eight-month-old daughter while she attended a medical appointment. I had worked a night shift and should have said no. I wanted to help. I put the baby in the playpen, sat on the sofa, and fell asleep. When I woke, she was crying on the floor beside the coffee table. Somehow, she had climbed out. She had a bruise on her forehead. The emergency room said she was fine. My sister keeps telling me accidents happen and that she should not have asked when I was exhausted. I hear none of that. I see the baby on the floor. I imagine every version where the corner of the table was two inches closer. I have stopped volunteering to babysit. My sister says I am punishing myself and making things harder for everyone. Perhaps. Last night, my niece reached for me when I visited. I could not pick her up. A baby forgave me before I had even decided whether I deserved forgiveness.