ESM4156 | Particular occupations: family employment - other than by spouse
From HM Revenue & Customs · Employment Status Manual
SS (Categorisation of Earners) Regulations 1978 (SI 1978 No.1689) paragraph 7 in Part III of Schedule 1
For NICs purposes, if someone is employed by any of the following relatives in a private dwelling-house in which both the person employed and the employer reside, disregard it unless the employment is for the purpose of any trade or business being carried out in that private dwelling-house by the employer.
father or mother
grandfather or grandmother
son or daughter
grandson or granddaughter
stepfather, stepmother, stepson, or stepdaughter
brother or sister; and
half-brother or half-sister.
The sort of employment envisaged is that of gardener, carer, cook, etc.