Reference for Gittin 169:19

א"ל תניתוה לאבא ולאביך ובמאי אילימא בנשואין אבא ואביך בני נשואין נינהו אלא לאו בזנות ולאבא ולאביך הוא דלא שייר הא לאחר שייר

[If he said she may marry anyone] except the husband of her sister, what is the law? Do we lay stress on the fact that now at any rate she is not eligible for him, or rather perhaps on the fact that possibly her sister will die and she will become eligible for him? — He replied: We have [the answer] in our Mishnah: [ANY MAN BUT] A SLAVE, A HEATHEN. Now [the excepting] of a slave and heathen also [should constitute a reservation] since they can become proselytes? — [He rejoined]: Conversion is not a usual occurrence, death is.

Jastrow