If they live in Bristol and have decent jobs and accommodation, it may well be worth staying put at the moment.
When I look at the figures on this kind of stuff, it seems to me that the only people who can afford family houses in London are those who bought long ago before prices rocketed, so who have little mortgages, or those who earn mega bucks. In order to move there from somewhere else, people need mega mega bucks in terms of earnings or huge deposits.
It strikes me that the people who have the smaller flats in London or the houses outside London and who commute in, afforded that, by buying BEFORE they had children....they struggle thought the years of childcare costs by either not working and having a much lower income, using family for childcare, or using up savings or possibly building up debt, until they can get onto a more stable footing once childcare costs drop or end.
The idea of moving to the most expensive city, from elsewhere, when you already have 2 kids, the point when your expenditure is at its peak due to childcare, will never be easy or possible for most people. Those people face the double difficulties of not having big enough deposits to be able to get mortgages down to a reasonable level, whilst also being burdened with massive childcare bills.....it's a killing combination.
OPs sister could wait. They could wait for childcare to end....yes, house prices will continue to rise in that time. However, 4 years down the line,the big expenditure will JUST be housing and at that point, a flat in London or more likely a small family house outside of the M25 but commutable could be an affordable option.