YANBU to be hurt and angered by it at the time. YABU to feel exactly the same way 23 years later.
That's why we have counselling, self-help, meditation, exercise, creativity, intellectual work and all the other things people do to help them deal with the shit life throws at them.
Best of all, we tell the people involved how their actions have made us feel, though that does take a certain amount of courage and confidence. Sounds like you don't have a lot of confidence, so maybe you're projecting your anger and frustration onto her, as well your hurt at the original incident. But as you've found, that doesn't work as it only harms you. She gets on with her life blithely unaware.
If you talked to this lady, you might find it all stemmed from a generational or cultural gap. Perhaps your MIL thought she was helping out, after you'd had such a traumatic birth and were recovering from losing so much blood, to put your parents' minds at rest. Nowadays we'd be outraged not to give the news ourselves - I would be too - but people used to be less precious about that kind of thing.
Equally, maybe she is just an interfering thunder-stealer who enjoyed nicking your big moment. Such people exist and they need to know that it pisses people off.
You've known her for more than 23 years so presumably have some idea which kind of person she is. Either way, you need weigh up whether you want to have that conversation with her. If not, you just have to close the book on it, for your own peace of mind.