פירוש על קידושין 34:18

A non-Jew does not inherit from his father, should his father have converted. Similarly, a convert does not inherit from his father if both of them have converted. The issue here is that the rabbis think of conversion as a rebirth of sorts—one loses all ties with one’s previous family. Therefore, if the laws of inheritance applied, it would be as if conversion is not actually a rebirth.
This is illustrated by a mishnah in Sheviit. Normally, if one’s creditor dies, one must repay his inheritors. But not if they are all converts. Indeed, the rabbis were not happy with one who did so.