Hmm. Apart from the 10 years younger bit, I could be your SIL.
But I can't help being the way I am. I can't help the fact that conceiving babies is really easy (shag once --> pregnant), and nor is giving birth (4 hours first labour, 2 3/4 hours second labour) - although I did lose a baby between those two.
I'm also very organised. It's how my brain is wired. I could no more be disorganised than fly to the moon.
I also have had - over the years - a career that has been much more impressive than my SILs, although mainly because she chose to be a SAHM and I have always been a WOHM. I make no judgements about that, btw - it was what both of us wanted, so we were both lucky that we got what we wanted. My SILs jobs, once she started working, were always about fitting around her DC so that no childcare costs were involved, and she has managed that very successfully. Now her DC are old enough, she has a good job, full-time - although it's probably not one that she might describe as a 'really impressive career'.
I love my SIL and (I hope) she loves me. But a few years ago we went on holiday together - my family and my brother's family, and my dad. And it was bloody awful. It was so clear that I made her feel inadequate. I was not trying to do this in any way, shape or form. She decided that she felt inadequate and couldn't 'compete' - and it was a horrible experience for me to go through as well.
You probably won't believe that, but it is absolutely true, and it spoilt my holiday - I spent the time trying not to be organised, trying not to make her feel inadequate, with the consequence that I had no idea how to behave on my own holiday.
Because I can't help being the way I am! (Although I tried bloody hard, I can assure you)
You say she is very competitive, but actually it does sound like you're the one trying to compete, in a situation where competition is totally unnecessary.
So I have empathy for your situation, although clearly I'm on the 'other side'!
Try and picture things from your SIL's perspective if you can.