Will referring him at this point of time cause a loss of my credibility or any other issues?
I think that the only way one's credibility can be harmed is if you refer a candidate that turns to be problematic, or that had really fewer experience than the way you portrayed it (and somehow managed to get hired before the lack of skill becomes evident).
Seems like your brother has decent/some experience in the subject, so even though there probably are better candidates it is not like you are referring someone who has no clue.
Now, the situation you have here I consider is a bit different than the way you are focusing it. I believe that given you only have 2 weeks working there probably any referral or recommendation you give now will not be regarded as highly as the ones of senior or more experienced coworkers.
It's not that your referrals or your brother are unskilled or poor choices, it's just that you are still new in the company, and saying "Hey would you hire my brother?" two weeks after joining them may not be taken too seriously (perhaps it could even give a bad impression).
Bottom line, unless your brother is a really good candidate referring him now may be ineffective or worst case harm your credibility a bit, so I suggest you don't.
Edit: So, if we consider that other question you commented:
How long should one be working before referring someone (generally speaking)?
There is no unique answer for this.
If you refer someone is because you think they may fit well in your company. Now, with only two weeks under your belt, I really doubt you know the company the point to be able to say if someone is a good fit for it.
You may know your brother and his skills well, but you are still lacking knowledge on the company and its specific processes and details to really know what person may be a good fit there. With time you will start to know better your way in your new job, and when you reach that point you will be better prepared to refer people.