By the Grace of G-d 24 Menachem Av, 5737 Brooklyn, N.Y.
Worcester, Mass. 01602
Greeting and Blessing:
I received your letter and will remember you in prayer for the fulfillment of your heart’s desires for good.
With regard to your question as to what can be done to improve the situation which has affected your parents’ health - generally, in the matter of what should be your parent’s relationship with your sister in all detail, here a ruling by an authoritative orthodox Rov is required, after giving him all pertinent information.
The point I wish to emphasize - which apparently you do not fully realize - is that intermarriage is not just a transgression, but one of the worst tragedies for both parties involved, not only the Jewish side but also the non-Jewish. Hence, first of all, there is no place here for any claim that it is a personal matter and others should not interfere, or attempt to rectify the situation. It is like a case of a would-be suicide, G-d forbid, who claims that his life is his own affair and he is prepared to take all the consequences and no one should try to stop him or discourage him. Obviously, every person nearby, even strangers and non-Jews, should make every effort to prevent it from happening.
The above is also an answer to your mentioning that the parents are not strictly frum - which is entirely irrelevant in this case. Suffice it to consider the statistics about mixed marriages which leave no doubt that it is a terrible disaster, not to mention the numerous cases which are concealed out of shame or other reasons. Of course, from the viewpoint of Yiddishkeit it is even worse, but even aside of that, considering the human tragedy involved, the greatest effort should be, even by non-Jews, to avert it.
As to the ways and means to do it - in accordance to the Shulchan Aruch - and which of them would be more effective, a great deal depends on the character makeup of the person involved, etc.
But one general point which is applicable in such situations is this: a Jewish family is like one unit, one body, where a benefit to one part is a benefit to all parts. Hence, when any member of the family, especially the parents, strengthen adherence to Yiddishkeit in the daily life, it benefits all the family, particularly the children. Clearly, the parents can do a great deal in this area, for in matters of goodness and holiness, Torah and mitzvos there is always room for improvement.
May G-d grant that you should be able to report good news in all above.
With blessing,
Notes and sources
Published: The Letter and the Spirit, Vol 3 (Nissan Mindel Publications, 2016), 324.
Note: Letters printed in "The Letters and the Spirit" are from the Archives of Rabbi Nissan Mindel A"H, the Rebbe's personal secretary entrusted with writing up the Rebbe's orally dictated responses. The “Nissan Mindel Archives” are comprised of secretarial copies, including first drafts, and may have subsequently been published with editorial changes. Therefore the letters as they appear in the book may not be the final signed version.