Elder can simply mean a sibling that was born before you but in common use elder refers to the oldest sibling in a group. Therefore we could see the following: 1. Tom. Aged 46. The eldest brother 2. Frank. Aged 30. Your older brother. 3. You. Aged 20. (for this example) 4. Lisa. Aged 18. Your younger sister 5. Sandra. Aged 16. Also your younger sister. Also your youngest sister. One would only say 'old brother' if we wanted to refer to his age, most likely in the form of an insult. Not used very often, even in this case. It is the same with 'young sister.' Though not commonly used we would only say this if we wanted to emphatically emphasize our sister's young age.

- elder is a little more formal than older, but they have the same meaning in sentence 1/2 - elder makes me think that you really respect that person, and/or they are a lot older than you - 1/2 show that you are related, and that your brother was born first; 3 does not show this - 3/4 sound unnatural and are not commonly used - 5 shows that you are related, and that you were born first; 4 does not show this - also, elder cannot be used for things, only people

Sorry I wrote my answer down there!