Hmmm, good answer. However, let's assume that your mother is a widow.

There are two answers. The clever answer is that if you're both children of Abraham and Sarah, you're technically siblings, and thus can't get married. Unfortunately, this also implies that converts can't marry one another at all, so the rabbis dropped this one after a while.

The real answer is, basically, that the Gentiles will get the wrong idea if we allow you to marry your mother. So while it's technically permissable under Jewish law, you shouldn't do it, because it would undermine the reputation of Judaism. Shows you that the rabbis are quite capable of being pragmatic when they have to be.