I tell my siblings they are making me broke with all the kids they are having; I never considered that they might find it anything other than funny. Thank you for that perspective!
having said that, the joke is like megan-alice93 says: not being able to afford so much stuff for them. I don't mean giving them stuff, because I can't out stuff the grandparents, but I'd love to be able to take them places like parks, short trips to lakes, rivers, oceans, etc (all within a 30 min drive of us) and for my youngest sister's kids, flying out to visit them a lot. Being on medical disability is not fun, but the money crunch is really inconvenient when it comes to having their birthdays piling up seemingly all at once!