SDP run on when carer moves in

How is the following interpretted for SDP run on purposes. CLient son has moved in to care for mum (she receives enhanced rates of PIP DL and mobility and lost SDP straight away) after living apart for many years. https://www.legislation.gov.uk/uksi/2002/1792/schedule/I/paragraph/2 states presence of the (son) can be ignore for the first 12 weeks , if

(3) ‘a person who joins the claimant’s household for the first time in order to care for the claimant or his partner and immediately before he joined the household, the claimant or his partner was treated as being severely disabled.

Does this mean that mum cannot have the run on because the son once lived in this household or should this be read as they can have the run on because it is the first time the son moved in for the purpose of caring?
thanks