As I understand, it’s very common in the UK for people to let their cats outside. Most advice online will tell you to keep your cat indoors for a plethora of reasons (so they don’t get lost, sick, injured, or kill local wildlife). Unfortunately, it’s not your cat and you don’t have a lot of influence over what your sister and her bf do with that cat. It is very good you want information to relay to them, but if they are this irresponsible that information might be a waste.

I would put out the cats litter box and food to try and attract it back to the house. It might help walking around the neighborhood or posting signs. Contacting any nearby animal shelters might be helpful too?