There can be no 'one-size-fits-all' answer to this question.
TV transmitter powers are set on the assumption that all viewers in the relevant area will have good quality external aerials. They always have and they always will be. It has NEVER been the aim of broadcasters to provide a usable signal to people who've only got indoor aerials. However people who live reasonably close to a transmitter
might be able to use an indoor aerial.
Where I live half the houses have external aerials pointing in one direction (to the Tacolneston transmitter in Norfolk) and half of the houses have them pointing in another direction (to the Sudbury transmitter on the Suffolk/Essex border). All of them though are set very high up (often atop of long poles) and are expensive multi-element types. The chances of getting a signal using a standard indoor aerial here (even if it's an amplified model) are absolutely NIL.
I've actually managed to get a loft aerial to work (after my rooftop one fell down and I couldn't afford to pay someone to put a new one up for me) but that was only after doing a lot of research to find a multi-element aerial (designed for rooftop use) with the maximum possible gain and then installing no fewer than three amplifiers between the aerial and my TV. If I'd have purchased a normal indoor aerial I wouldn't have been able to receive anything at all.
So, unlike some others above, I won't rush to say that your sister could get away with using an indoor aerial; only a small minority of TV viewers can do so.
The vast majority of aerial fitters (I'd estimate at least 70%, and possibly far more) are absolute cowboys who know nothing about the job and who couldn't tell a Yagi antenna from a patch one, yet alone calculate their gain figures. Anyone with a ladder, a drill and a screwdriver can do the job, albeit often extremely badly.
Your sister should seek out an installer who's CAI-certified, which will vastly increase her chances of getting someone who turns up to do the job and who then does it professionally:
https://www.getmeviewing.org.uk/