I don’t think it’s even necessarily an issue of being child free vs parent but simply having a bit of empathy.
None of my friends or family would watch me go through something challenging in my life, and say “Haha, glad that’s not me” or react to me dealing with some unavoidable bodily function by saying “That’s disgusting, Urgh! Glad I don’t have that on me.” I mean, how insensitive do you have to be ??
I’m trying to think of examples - maybe if I told someone in mothers group that my baby was waking 6 times a night and she responded “Haha! I’m glad I’m not wasting my time doing that! My baby sleeps 12 hours and I’m not even tired!”
or if my boyfriend saw me dealing with menstruation and said “urgh! That’s disgusting! I would hate to have blood on me!”
In both cases, if someone carried on like that I wouldn’t even bother to try to explain to them how their words affected me, I would just have moved on and found someone more empathetic to be friends with or to date.
Of course privately you might be relieved that you’re not facing the struggles someone else has in their life, but how much of an insensitive git do you have to be to actually say that to their face??
The only reason I suggest trying to explain to SIL is that she may just be so self-centred that she doesn’t even realise the impact her words are having and I think family relationships are important and we should try to work at them.
Also, it’s natural that having wanted and now not having children, she will be trying to psychologically protect herself by noting and emphasising the downsides of parenthood. That’s normal and healthy for her. But she needs to understand that it’s best to keep these thoughts to yourself, or at least discuss them with someone else!
So I would give her that opportunity, whereas with a random person I wouldn’t even bother.