I recently finished reading Mothers and Others by Sarah Blaffer Hrdy. It's a book about how the human species would never have survived if we didn't have alloparents (like aunts and grandmothers) to help care for, feed, and keep safe our slow developing offspring who are dependent for years after it's time to birth the next one. It's my belief that your nieces and nephews will cherish all the memories you made with them and the things that you did for them - but only when they are adults, because kids are entitled little beasts who seem to believe that it's just natural and what's expected for us to do all these things for them. But (hopefully) someday they will appreciate having had an aunt who was close to them, loved them, and gave a lot of herself to them.