Complex sentence: Katie's insecurity lapsed considerably once she was welcomed into the stable family environment made friends with Toby Mangel, Jane Harris and Helen Daniels, who also lived at the Robinson house.
Split into simpler sentences: Katie's insecurity lapsed considerably once she was welcomed into the stable family environment of the Robinsons. She made friends with Toby Mangel, Jane Harris and Helen Daniels, who also lived at the Robinson house.