וְלֹ֥א תִתְחַתֵּ֖ן בָּ֑ם בִּתְּךָ֙ לֹא־תִתֵּ֣ן לִבְנ֔וֹ וּבִתּ֖וֹ לֹא־תִקַּ֥ח לִבְנֶֽךָ׃
neither shalt thou make marriages with them: thy daughter thou shalt not give unto his son, nor his daughter shalt thou take unto thy son.
Read kabbalistic (mystical) interpretation of Deuteronomy 7:3: and classical mystical sources.