They agreed to look after Rebecca for "a few days" over Christmas because Granny Margaret was "unwell". Since then, she seems to be just "unwell" still, so Rebecca hasn't gone back yet. Margaret initially asked Pauline if they would look after Rebecca for a few days, and Pauline agreed (without consulting Martin & Sonia) becasue she realised that it would help to keep their marriage together. I think the whole storyline is a bit silly, unless they are planning to let Margaret die somehow.