It is not unusual for there to be adults, apart from the foster carers, in the fostering home.
There may be the foster carers’ grown-up children or young people who have remained living with their foster carers after turning 18 (and they may have partners), or a foster carer may have a new partner.
The usual expectation is that when there is an adult living in the fostering home, or visiting the home regularly, they will undertake a criminal record check. If the adult was living in the home at the time of the initial foster carer assessment, that will be taken into account as part of the assessment.