This was asked years ago, but nobody responded to the previous posts. Likely things haven’t changed much, but I was just curious if anyone knows about whether a sibling can be added to an ARC that you acquire through work?
Nope. Only spouses and children.
Though maybe there are special cases, if you are your sibling’s legal guardian or something like that? As usual, it’s better to check with the competent authorities than on online forums.
OP, you can start your research here
https://www.immigration.gov.tw/mp.asp?mp=2
For Foreigners In Taiwan: Domestic 0800-024-111, Overseas 886-800-024-111.
If nothing shows up, I’m sure calling them is probably the quickest way to answer your question.
Thanks for the link! I’m not sure the authorities have actually written a clear answer, so I’ll likely have to end up calling.
I understand calling and speaking to someone is the best option, but when you get different answers depending on who you call it can get a little frustrating 
(and that’s on all legal matters, not just visa issues)
That’s a good and bad thing about Taiwan immigration laws, they are always changing. You also have to factor in that almost no one’s case is exactly the same and can be interpreted differently. Also the regulations change so often that by the time personnel understand the ins and outs of the law, it’s already changed. So you can’t really blame them (too much).
This thread is somewhat relevant.
http://forumosa.com/taiwan/viewtopic.php?f=66&t=83553
Even if you were a Taiwanese citizen, it probably wouldn’t help you. So being on an ARC or APRC probably won’t help for bringing a sibling. If your sibling can work, then that would probably be the best way other than for your sibling to marry a Taiwanese citizen.
I feel your pain. It’s always preferable to get an answer in writing, though that can take time.