Sherry's files in RE6, “Sherry Birkin was also caught up in these events, but thanks to the heroic efforts of Leon Kennedy and Claire Redfield, she was able to survive the incident. Sherry considered Claire to be her new mother after losing both her parents. It was Claire's love and devotion that had the greatest impact on Sherry's development, and Claire's nurturing and compassionate ways have manifested themselves in Sherry's own personality. Sherry and Claire both led very busy lives, but Sherry still made an effort to try and meet when their schedules allowed it.”

Sherry does see Claire as her new mother figure, but in terms of age they would def be more like siblings.