If one’s daughters married non-Jews, can their children and spouses attend the Pesach seder according to Jewish law and custom?
Your question is a very interesting one. Let me first respond to its content, and then to its context. Both are important.
First, there is no halachic (Jewish legal) impediment to the presence of non-Jewish people at a seder. You may be aware that, in the Biblical era and during the period of the second Temple, only Jews were to partake of the paschal lamb, which was to be sacrificed on the eve of the holiday. But virtually since the destruction of the Second Temple in the year 70, Jews have not sacrificed an actual paschal lamb on the eve of Passover and do not eat one on the holiday, so there is no reason that non-Jewish people may not be present at and participate in a seder.
It is true that Jewish communities vary regarding the propriety of the presence and participation of non-Jewish people at the seder. In some it is less common and even frowned upon, because the particularistic aspects of the Passover tale are emphasized. The liberation of the Jewish people from Egyptian enslavement is seen as an essential -- and essentially Jewish -- particularistic responsibility. There is a greater focus in such communities on what liberation has meant and continues to means to “us,” -- meaning, to the Jewish people. Because non-Jewish people don’t share a Jewish identity, the story is not theirs, and therefore they can’t fully participate.
In other communities (such as my own), the universalistic aspects of the Passover story are also emphasized, and it is common to invite non-Jewish people to the seder. In fact, it is seen as an enhancement of the seder experience. After all, redemption from slavery to freedom remains an urgent universal concern—as well as the particular heritage of the Jewish people. Hearing and reflecting on the stories and perspectives of non-Jewish people enriches the experience for all present, both Jews and non-Jews.
Many years ago, I invited a Chinese friend of mine to a seder at my home. This was just after the reign of Mao Tse Tung. Our guest described what he had endured during the so-called “Cultural Revolution”. It was riveting. He had been separated from his family, and had had to endure hard labor and much suffering—and for several years. In the Bible, we are told that the Egyptians oppressed the Israelites “b’farech” (“with rigor”). My friend’s story helped me and the others present at that seder better understand the meaning of that word.
Now, as to the content: If the “one” in your question is you, then you are asking a very personal question. You’re not asking whether some generic non-Jewish person can be invited to some generic seder; you’re asking whether your own sons-in-law and your own grandchildren (who, surmising from your question, are possibly not being raised as Jews) can be invited to a seder—perhaps being held in your own home.
To this question there is a clear and definite answer: yes. A Pesach seder presents an opportunity—an extraordinary one—to share your understanding of Judaism and your appreciation of its traditions and values with members of your own family. How could this not be an important opportunity for keruv (drawing near) the non-Jewish members of your family?
Your question suggests uncertainty regarding the propriety of sharing one of the most intimate of all Jewish observances, the Pesach seder, with your daughters. Why? Because they have chosen to marry non-Jewish husbands? To me, it is hard to think of a better time of the year to reach out and to embrace them, their husbands and their children.