People do have such different ideas of what an acceptable or comfortable standard of living is.
For some it needs to be a 'family' house with a garden in a good area and with good schools and money spare for holidays abroad and a new car every three years. People who bought in London 15+ years ago might achieve that on incomes of less than £100k which were considered quite standard, mostly because their mortgages are now relatively small, so those outgoings are small and because if they bought back then, their children are probably not of a a ge to be in full time childcare.
Those looking to start in London now won't achieve that same standard of living, in terms of family houses unless they have somehow got a big deposit or earn loads.
So most families who are starting up on the property ladder for the first time in London don't have that standard of living...it is flats, and managing workloads and hours to minimise the amount of childcare needed - quite different....but becoming the norm for an increasing number. Being in a flat with children sharing bedrooms is no longer seen as odd or a sign of poverty, because more and more people ar doing it....and are quite happy with it. They might like a garden, but life can still be good without one.
London is unusual. In other areas, even those in commutable distance to London, 2 decent salaries of around £60-70k in total will be enough for the house, rather than flat. And because most of us probably grew up in houses rather than flats, it is one of the reasons why people still feel having a flat with children is somehow a poor option or failure to provide properly.....times have changed and it is perfectly the norm now.
People living in London are right that they can feel not especially well-off,with earnings of £100k due to the cost of housing etc. They might not have loads left after costs of living......but they should still recognise, that this in itself doesn't make them poor - with earning of that level,they are certainly still in the top 5% of incomes in the country and recognising that fact,regardless of whether they feel rich or not is important. It shows a total lack of empathy for others and the choices (or lack of choices) others on lower incomes have,to fail to recognise it, or to say that the only acceptable standard of living is one of a family house, with a garden and private education and holidays abroad, in London,mor anywhere as a matter if fact.
Because the OPs sister has a deposit of £100k, the figures upthread suggest that the income of 75k is likely to be enough to get them a flat in London or a house outside of the M25, especially if they can bear an interest only mortgage until the childcare costs end. So it IS doable, if they can tolerate living in to flat or having longish commutes. And that is what loads and loads of people are doing now - having a flat or a longer commute. It might also be doable down to about £60k, but much below that.....to buy, to travel to work and to pay childcare might not be possible...of course if one of those requirements such as travel costs or childcare costs can be reduced or got rid of, lower incomes still might manage.
We could say the Ops sister is 'lucky'. She has a deposit of £100k which many with young families won't have, because they have never owned before and haven't been able to save much. They also have a family income of 75k and 'only' 1 child needing childcare. Many others of a similar age with 2 children wouldn't be able to take the London job because they would need to put 2 children into childcare and haven't got a deposit sufficient to get a mortgage big enough or to service it monthly. It is those who have even more limited choices.