Checked and the tds at my sil's is 173 and mine is 285
That's the problem up here in North Wales, you never know where the water is coming from. My TDS is around 57ppm, in Prestatyn 5 miles away it's near 200[so I've been told], and 25 miles away in Llanduno it's 42.
Ask your sister if she has any mates 5 miles away, their TDS may be a lot less.
You would still get away with DI only, it's only for one job.