I would probably be like, "Wow, that's four years from now, isn't it? Hasn't even been on my radar!" in a cheerful tone. I also like the response of, "We're planning to do whatever my parents want," because you are
, you just have no idea what that is.
As far as your obligation, as others have said, there's not one at all. I would suggest asking your parents if they've given it any thought at all, since Aunt has asked. If it turns out they would
like a fancy vacation or something, the sooner you know about it, the sooner you can start saving up, if you indeed wanted to contribute money towards it. But people have a lot of different ideas about how they want something like that celebrated, and I think the most important things are to listen to them, and then work within your means. If they don't want a big party, then don't push one on them; on the other hand, if they want an all-expense-paid trip around the world, that's nice, but you and your sister aren't obligated to provide it.
For their 40th anniversary my aunt and her husband had a big party at a church hall and invited all their friends and family, including all the members of their wedding party. For my parents' 40th anniversary, which I think was just last year, my parents went out to dinner at a new diner in their small town. Little bit more low-key.
I've never had a tradition of doing anything for my parents' anniversary, actually. I suppose if they came to me and said they wanted me to plan something, I'd consider it as far as my resources allowed, but it would be a very, very unusual thing for all of us. Definitely a third
party who expected me to do something big would get a