Hi Andrew,
Thanks for joining us. You can't control getting MS and you are young and need to live your life. I think you may already know where to leave your fears :) just need a reminder.
Ok, so a little help convincing is order. Being male, your risk already lower than the majority. I hope that makes you feel better. Also, these are your 1/2 siblings, decreasing your odds even further. I don't believe it's the same stat as sibling or twin.
You have read a couple good stats especially ref the 1 and 800 general population ratio. At your age, smoking should not make a difference at all. I was dx'd older than that and smoked longer than you. It's ok - but, quit if you can while you are young because as you mention it's not good for MS (or anything for that matter). It's nice to see you did your homework and happened upon some solid information. There is a lot of bad information on the www and MS is not excluded.
I'm so sorry for what your brother has lost and it's reasonable for you to see that and be fearful. How are his spirits?
In the end, I don't think your odds are higher than anyone elses here. And, if you should be among the population who gets it, don't fret. Disease modifying drugs have changed the course of MS for many. And, we're still here is that is any consolation to you.
This discussion may help too:
http://www.medhelp.org/posts/Multiple-Sclerosis/Genetics-of-MS/show/923576
see you around,
Shell