You shouldn't be sad at all. I believe that both parents have poor conditions to take care of their children, and your sister has a house without marriage. You can find better conditions when you get married in the future. As a sister, you should support her. If your parents have no money, as an elder sister, you should raise a dowry for your sister. So you should be glad that your parents are rich and lighten your burden.
As far as my family is concerned, I am in the middle, so my sister and I help them buy a house. I also repaired my parents' house, and I also bear the old-age insurance alone. Usually when my parents are sick, I pay for it alone, and they contribute. I think this is very good, and being needed by my family is also a kind of happiness.