You're taking a risk by hiring them. You could fire one and lose the other one or reduce their productivity if they get angry. I'm sure you realize that, hence the question. Ask yourself these questions:
The end, summarized idea is that if you can avoid it, do so. Don't hire as a favor to your current employee - don't risk a big blow to your business, and your employee should understand it. If you need to or want to for whatever reason, make your best guess on it and make sure you make your expectations clear to both, lay it right out there - you expect professionalism and each to respect your working relationship with the other, including your ability to terminate them if you feel it's in the best interests of the company. It's all in your judgement. Good luck with whatever you decide! Hope this helps!
It is hard to find very good people. But bear in mind that if you fire one of them or have problems, are you gonna be able to maintain a healthy professional relationship with the other one ? On the other hand, if it sticks well, you might have that extra power.
There are evidently two schools of thought on this, but I say Don't do it. On top of the points mentioned, the other employees may look at the situation negativity and may be jealous of the fact that the sibling has the bosses ear more then they do. The boss will be in a tight bind, having to choose between taking the siblings input into account versus the other employees input and may then actually give less credence to the sibling as to create a false balance. This will create a TON of animosity, especially if the input is perfectly valid.
I went through this on my team, it's not worth it.