Q. Can one invite a totally irreligious relative, that one wants to be mekarev (bring close to Judaism) to the Pesach Seder?
A. Shulchan Aruch and Mishna Berurah (512 : 1) rule that one should not invite Gentiles on Yom Tov for a meal, since we are concerned that one may cook additional food for them which is prohibited. Mishna Berura (ibid 2) mentions that an unbeliever or one who does not observe Shabbos is considered like a Gentile and cannot be invited.
However, Chazon Ish (Y.D 1: 6) maintains the ones that were raised and educated in non-religious are considered like Jews. (See also Sdei Chemed (40: 86), Piskei Teshuvos and others).
Tzitz Eliezer (8: 17) permits when everything is cooked before Yom Tov, similar to the way we do on Shabbos,
Horav Shlomo Miller’s Shlit’a opinion is that if you inform the guest before Yon Tov that you cannot cook anything special for them and they would be sharing with you the food already prepared, it is permitted especially since the intention is kiruv.
Rabbi A. Bartfeld as advised by Horav Shlomo Miller and Horav Aharon Miller Shlit’a