CV on dogs' fur

The virus can remain on any surface for different lengths of time. This includes your coat, trousers, shoes etc. and of course the dog.
Assuming you take the dog out then it can (and you can) being the virus into the house.
That is why staying home and not allowing anything that has been outside into your home without cleaning it with soapy water, 60%+ alcohol or diluted bleach (not anti bacterial stuff) is potentially risky.
You need to wash your hands thoroughly every time you touch the dog unless it has been inside for days.
Personally I'd return the dog to your sister.

Don't worry about the dog. he's the last of your worries. We're walking ours once instead of twice a day. Ours are generally on lead because the park and open spaces where they can usually run are of course, much busier with people usually at work. This has meant an influx of dogs we don't 'know', sadly many off lead and with owners who haven't trained them to recall. This means our dogs are on lead to avoid any issues that mean people coming to close to us.
No-one is stroking our dogs, or getting close to us.
We are being careful but there's being careful and being a bit obsessive and overly anxious. Let's live our lives as best we can during this tie.

Aren’t we all in danger of spending what may be left of our life wrapping ourselves up in a prophylactic and bleaching anything that stays still long enough?
Self isolation without some light relief is like one of the seven circles of hell.
I stroked my son’s dog yesterday. He was going to bath the dog afterwards and I washed my hands thoroughly. But we all had fun. The son lives next door and we kept the 6’ apart.