Some people might say that it is the right to be given whatever you need to continue living. But that can't be right, as Thomson makes clear by another example. It would be very nice of her to do it, but she does not violate your right to life if she refuses. You have given the violinist no right to the use of your kidneys. So you don't violate his rights if you refuse to let him continue using them.
Nothing follows yet about abortion.
But on the general issue of rights, Thomson offers another approach, in terms of justice. It's plausible that the right to life is the right not to be killed unjustly. And if this is right, it is hardly clear that all cases of abortion are cases of unjust killing. But if they belong to him alone, he is not being unjust to his brother, even if he might be mean-spirited. The brother has no right to the chocolates.