The clues were there from the start about Clare's deceit - she didn't tell Catherine when Tommy got out of prison because she didn't want to upset her.
That's another layer of interest, Clare and Catherine's relationship. Clare is often nervous around Catherine. When Daniel moved back in after splitting with his wife, Clare felt she should give up her room as "Daniel has more right to be here than I do" or similar words. Clare also voices how Neil is frightened of Catherine, which Catherine irritatedly dismisses. I think Clare was speaking for both of them!
Catherine could not have continued in her career in the way that she had, working shifts as well as being a single grandma, if Clare had not been the stay-at-home parent in effect. At the end when Catherine says to TLR that Ryan is a prince, that will be in no small part to the gentle, stable and loving care that Clare has provided him with. Hopefully Catherine had time to reflect on that. Even at the end (when still cross with Clare) and Ryan said Clare had always been there, she rather spitefully said Clare had nowhere else to go. Still, that's why I loved HV - it wasn't saccharine. Catherine would never have suddenly done a sugary speech about how Clare meant the world to her, best sister ever blah blah.