Complex sentence: Kate and her sister Nora were still quite young when their widowed mother moved her little family from Philadelphia, Pennsylvania, to Portland, Maine, then, three years later, upon her remarriage, to the little village of Hollis.
Split into simpler sentences: Kate and her sister Nora were still quite young when their widowed mother moved her little family from Philadelphia, Pennsylvania, to Portland, Maine. Then three years later, upon her remarriage, to the little village of Hollis.