If working for a care agency, is it acceptable to ask the agency if I can provide sit in care for a relative?
It is not 100% clear what is meant here. Are you already working for the agency and is your relative already receiving a service from the agency? If you are independently volunteering in your own time, then that is a choice between you and your relative.
If you are asking the agency to send you as a paid employee, this is not a straightforward one to resolve as there are potential conflicts of interest that could arise. You need to consider if you would provide your service in the same way as another member of staff would. Once your relative becomes used to you going, they may not want another carer to attend. Alternatively, they may wish to have some variety in who they spend time with and have an alternative carer, but not want to risk upsetting your feelings.
There are many dynamics at play here, without knowing the situation in full, it’s best to consider all the possibilities and if you have concerns, speak with your employer.