Yes, a long commute is normal, as is living in a property smaller than you'd get elsewhere.....but lots of people are doing both, so people don't feel especially deprived or hard done by..they just accept it as the norm.
For the OPs sister, as for everyone else who has to choose between moving to London or not, if they can at least afford to live somewhere commutable and make the numbers work, is whether the higher income earned in London, or the experience of London living, makes the commute and/or smaller property worth it. And people reach different conclusions, with some deciding to go for it, whilst others decide to stay where they are. Neither is right or wrong.
One thing I you'll say to the OPs sister, is lifestyle aside and thinking purely about capital growth on property, if they can afford to do it, they are likely to see more growth in value on a London property over time,than elsewhere. So, yes it will hurt,me specially in the early days when childcare costs are high....but the values will only go up in the long term, so it could be viewed as a good investment.
I suspect prices will continue to rise so much, that a thread similar to this will run in 10 years. By then however, we will be saying that whatever the equivalent of 75k is then, WONT be enough anymore to get even a flat further in and the commutes necessary to make it viable will be significantly longer......so sadly, it is going to get worse, not better.