Bonsoir, I hear your testimony about your experience and that of your sister. My uncle's life has been similarly damaged after being sent up to Cambridge at the age of 16 (ridiculous). But you need to bear in mind that, for some infants, being placed in their default year would result in an experience very similar to the one your sister had.
If DS2s' secondary school were to make him jump 6 year now, we would again end up with similar problems to your sister. Does that mean I should have sent him to school at 4? No, I would not think so for a single moment. Because at 4 we bump into an absolute fact - he couldn't access the curriculum. So you can't project to 18 and work backwards with a child. You have to look at 3 and 4 year olds and accept that some of them are not at a stage where it is in their interests to join the default year - they need more time.
To put it another way, the "bored" able September-born problem (that you and your sibling faced) is totally different in kind to that of the 3.11 year old August-born who will not be able to access the reception curriculum in a month. The September-born's problems would be solved by excellent teaching and adequate funding, whereas bumping her up a year is a cue for social struggles (which I think is what you are indicating about your family's experience and certainly has been in my uncle's case). The 3/4 year old who cannot access the curriculum yet but might still even out needs their peer group adjusting, and they need time.
Thankfully, DS2 should be able to stay in his offset year provided we remain in Bradford LEA. He has no idea that we made a decision about his year-group. He wasn't conscious enough at 4 to have any sense of what was happening. Socially, year 2 is his correct year - his skills fall roughly within the year2 level, not within the year 3 level. But we know that this was the right decision because, after the "extra" year (or extra 11 days, really), he has kept pace with his adjusted peer group at all times.