My sister has taken in an apparently homeless cat just before Christmas (she was desperate to get in the house and wouldn't take no for an answer

) - she was wearing a collar so we have done the usual things to try to find her owners: taken her to vet to be scanned for a chip (nothing!), logged her details at various veterinary practices, informed the local authority, carried out a door-to-door / leaflet drop around the surrounding streets, put posters up on neighbourhood lamposts and in newsagents' windows etc, put a status on Facebook etc, but no luck so far

. She has taken it to the vet this evening who thinks there is a possibility she might be pregnant

and has said she is also covered in ticks which might point to the fact she has been 'out on the street' for a while - poor little love. If no owner comes forward, my sister will be happy to keep her, but would obviously want to get her vaccinated, spayed and chipped. The vet didn't want to start the vaccinations today because of the question mark over the pregnancy (she things it would be maybe 4 weeks), and my sister is now wondering what to do for the best. I hate the thought of more unwanted / unplanned kittens being born, but would the poor little cat suffer any physical / psychological issues if she was spayed and had her pregnancy terminated. The vet has said it is my sister's call. Of course, there is also the (slim?) possibility that the owner might come forward ......
Does anyone have any views on this, as to the best thing to do?
Sorry for the rambling and thanks for any thoughts you might have.
Vonney x