Dealing with your DC being negatively compared to others

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DC has a cousin who is 6 months older. MIL keeps making lots of snipey comments about DC's development when comparing it against that if their cousin. It doesn't help that the cousin seems to do everything light years ahead of any other baby.
I know that children develop at their own pace etc. But I'm worried this will be an ongoing thing forever. I don't ever want him to feel lesser than cousin, or like he's 'not as good'.
DH called her out on it once when it was not helping my pnd, but it's started to creep in again.

Any polite ways of dealing with this? I don't dislike her or want to make an enemy of her, or even restrict her time with DS.
Any ways of making sure my DC grow up with confidence and faith in their abilities?

Am I just worrying too much??

"Every dog has its day" on repeat. It will help when the DGC have developed their own personalities, I suspect...your MIL will (hopefully) compare them less and treat them as the individuals they are.

Thing is, their personalities are already emerging. One is full of beans and always in the go, one is more laid back and loves to cuddle/be read to/sang to. Even at this age, it's clear they are going to be very different people...

I'm sorry, we don't do comparisons, I find it's rude...

Same here, my ds has a close in age very advanced cousin. It really annoys me. My standard responses are 'that sounds like favouritism, it's a good job we love them all the same' and ' wouldn't life be boring without differences'. And I point out all the adorable things my ds does.

I have used the it's not nice to compare line. It hasn't completely stopped it but it has got better and I have learnt to ignore it mostly and limit the time in their company. As my kids have got older they have noticed and made comments which I leave the person making the comparison to answer... They normally stop

My exmil was like this. Her other dgc wore halos and carried a degree certificate from birth it seemed. Thankfully sil edged us all out of having any relationship with the ils when her dc continued to arrive and we never had to witness it anymore.
Back away op, it's not fair on your dc once they are old enough to realise.