I don't think it's so much that younger mums are resented, than that they are either disapproved of or pitied. It's just no longer 'the desired norm' to have children in your very early twenties, or late teenage. It's not 'right or wrong' in itself, just the way society has changed over the last couple of generations, thanks, basically, to women being allowed to have careers as a norm.
I would suspect a lot of women like me - not quite the first 'liberated' generation of the sixties, but the next wave in the mid-late seventies - think that it's a 'seriously bad idea' to have children before you are about 30! It was drummed into us that AT LAST we were 'allowed' to have pre-marital sex, simply because we were 'safe' from pregnancy. So, ironically, whilst it was 'fine' to have sex (as much as one liked really - though I also suspect most women, like me, had a lot less than we thought everyone else was having!! ) , it was 'not fine' to have children....it was almost as if we were behaving in an old-fashioned, pre-liberated way, if we had a baby.
So I think we 'disapproved' of young women having babies, as it meant they were unliberated. And we pitied them if they did get pregnant, as they were then 'automatically' chained to a pre-liberation lifestyle which was seens as boring and draggy....
Obbviously, the downside of that attitude was that a lot of women in my generation and the next one on simply left it too late - we ignored the realities of nature, and thought we could get pregnant on demand, when the time was right in our career, etc etc etc.
However, when it comes to your mum, I suspect that it goes even further back than the sixties, but into the fifties where, as ChrissiFi points out, having a baby out of wedlock was 'not done'. She has probably felt this way all the years since you told her you were pregnant, but has, maybe, up till now, managed to guard her tongue - now, with the stress your sister is under, that guard slipped, and out came 'the truth' that she has harboured all these years - that your getting pregnant was 'wrong'.
I agree with the others that it's best not to dwell on it overmuch. After all, most of us have mums who disapprove/d of a great deal of what we've done with our lives (!), and providing we don't let it upset us too much, there's no harm in it.
As for yourself, you've clearly made a complete success of your life, you've got three smashing kids who have grown up well, and a husband who is your life-partner. That's some achievement!
All the best, Julie.
(Thinking about it, by the way, the fact that your sister left it so long to try for pregnancy may possible be associated with an 'aura of disapproval' she may have picked up from your mum about your early pregnancy - ie, she picked up that 'you shouldn't get pregnant like that, young and unmarried' and let that influence her own decisions about having children....???)