If I have several older brothers, how do I refer to "my oldest brother" in Hebrew ?
In Hebrew we refer to the oldest among a group of siblings as "הבכור" (ha-bechor). So "my oldest brother" will be "אחי הבכור".
Can't you just say "watches me" (as in "keeps an eye on me") instead of "watches over me"?
He is a bit too much hairy. Sort of atavism. But i do not hold any prejudgies. :-)
Older brother would be the best translation, but elder is just not used in colloquial speech, at least not in the U.S.
You misread your own source. When it says "they also tend to connote seniority" it is telling you 'elder' tends to be used as a comparative. For example, to say the brother who is older than I, "my elder brother". I can say my elder brother when I am 10 and he is 12. This has little to do with "he's a senior or elderly". Re-read the source you quoted. BTW you are good to look for sources like this. English is a broad language with many meaning we don't experience in our own lives. This is why I always double check my grammar before posting, and why I appreciate that you did too.
In reference to seniority, then they give examples that are NOT about siblings. you'll note that all the sibling examples use eldest or older, not elder.
If you're elder you might use it for siblings but it's just not common today and is NOT the preferred usage.