I am part of a large family. Most of my siblings had children years (and in some cases decades) before I had mine. I have bought Christmas presents for them every year, even when I was a student and had no money. We are talking more than 10 children.
I had a baby last year. Come Christmas I made sure to write down gifts and who bought what so we could write thank yous. There was nothing from my sister. Apparently she asked me what we wanted for him, I never got back to her so she just didnt bother. I don't remember this conversation but if she asked she certainly didnt ask a second time.
I have asked people for suggestions every year. If they dont get back to me i buy whatever i think. I actually much prefer not asking or knowing as it's nice to have a surprise.
Would I be unreasonable to not buy for her children next year? I haven't had a thank you or anything this year. I'm not trying to be childish, I get that present buying gets out of hand, I'd have just much rather her say 'I'm not buying for your son, dont worry if you dont want to buy for mine'.
She didnt get him anything at birth either (I didnt expect anything, just to be clear that there wasnt a 'maybe that counted as his xmas pressie').