First of all, congrats on discovering all this at 17yo and not 31yo like me, or even much older like some of us on here
Secondly, you having continued association with your mother and/or brother wont result in you being disfellowshipped so please carry as normal with them. Your brother in particular will be a good help to you. I myself have helped my younger brother out of the cult only recently. If you haven't already, download yourself a copy of Shepherd The Flock Of God (the elders guidebook that they keep secret from the congregation). A simple google search will bring it up. Or PM me and i'll email you a copy. It's definitely worthwhile knowing their rules and tactics they use to try and disfellowship people.
I understand that your position in that your grandparents will want you to attend meetings. I'd say continue going until you're 18 but do the bare minimum. No answering up or anything. Same with ministry, if you can avoid it - do.
Lastly, most of us on here have downloaded and read the book written by ex-Governing Body member Ray Franz called Crises Of Conscience. Do yourself a favor and read that, it'll open your eyes. Again, a simple google search will bring it up or you can PM me for it.
Please post back on this forum and let us know how you're getting on. This forum can be a real lifeline and we have ex-elders, bethelites and CO's on here that can help you.