Being envious means you want what someone else has and jealousy means that you're worried someone will take what you have. Being jealous of someone doesn't necessarily mean you're envious of them, but these feelings often go together. While these are the technical definitions, a lot of people don't make that distinction and use the words interchangeably anyway, so don't worry too much. Ex: I'm envious of her fame. You aren't jealous because you don't have any fame that she's able to take from you, but you envy her because she has fame that you want. Ex: I'm jealous because of how much time she spends with my boyfriend. Here is a situation where both definitions would be applicable in their own way. You're envious of the time she is taking from your boyfriend and want it to be yours and you're feeling jealous because you worry you'll end up losing your boyfriend.

Elder and older both mean "advanced age," so I see where the confusion is. In the example phrase you have listed, elder and older would be able to fit interchangeably. A key difference is that elder is used only with people while older can be used for anything (animals, objects, etc.) Ex: "The building is getting older." You can't convert this to "the building is getting elder." Elder is viewed as a more polite, formal way to say old, generally. Someone who is very old and wise would be called an elder. On that note, elder can be used as a noun while older can't. Also, elder can't be used to compare things like older can. Ex: She is an elder. You can't convert this to "she is an older." Ex: She is older than him. You can't convert this to "she is elder than him." If you have any other specific sentences/phrases you'd want to review about the usage of older vs. elder I'd be happy to help. This is just a quick explanation.