ah well, i only have dd and not much chance of another one.
but, i am borderline ap and endured much critisicm in the first 18m for molly coddling and making rods for my own back.
those same people are full of praise now for how well dd sleeps, eats, takes turns and shares etc.
tbh, it's her nature. she's really at heart a very pleasant child.
now, she has several siblings two of which are utter classics. sds2 was and is a delightfully easy child. sdsd2 was, apparently, a complete sleep/eat nightmare to age 3ish. totaly supernanny material and no apparent reason for it.
i guess, in (not so) short, i am trying to say that each child will be different and it's very easy to be jealous of easy sleepers/eaters. i know i was when dd was still co-sleeping and feeding every 2 hours at 15m.
life is like a box of chocolates, you never know what you're gonna get.
you just deal with your child's needs as best you can.
good luck with your next, either way your SIL isn't justified in being rude but she probably is sleep deprived.