The celebration of the Jewish version of the Passover Seder meal, as far as I understand it, would be a sin if one was conscious of the fact that such a celebration was meant to be interpreted as a longing for the Messiah or Promised One, Who has come and can be found in any Catholic Church. If it pretends as if Christ has not come, then it is tantamount to a denial of Christ, Who is God. A Christianization of this tradition is, of course, harmless.
Christ, being God, is due all our love with all of our heart, mind, soul, and strength. But from what you are telling me, it would be a participation in a ritual celebration of another religion that, in its practice, denies Christ is come. There is a difference between family customs and traditions on one hand, and the ritualistic celebrations and enactments of religion on the other. Again, Christ Jesus is God, and if we imagine to do or offer anything to anyone save Him, we do appear to deny Him in fact and deed : Christ is “the Wisdom of God,” the very “Word of God.” He is the Second Person of the Blessed Trinity, inseperable from God for and from all eternity, reigning at the Father’s right-hand in Heaven, being the very “Arm of God.” The bread speaks of His Body, and the wine of His Blood.
Now Catholic writers have been very careful and tender as regards familial bonds and relationships, and have always distinguished betwen obligations due to family and secular community (e.g., work and civil relationships), giving much more lee-way for familial bonds than anything outside the family context ; however, it appears you are not bound to this anymore, being now married. It would follow that your first duty is to Christ your God, and nowhere have I ever heard of any precedent being established that permitted us to deny Christ, implicitly or explicitly, for the sake of traditions or family.
You are a daughter of Grace, another Mary, a lily of Israel, and your citizenship is in Heaven, and your discourse there also ; your Passover Seder is of the New and Eternal Covenant, containing Christ Himself. This one promises you Life Eternal and the joy of Heaven, and it establishes you as an heir in the House and Family of God, with direct recourse always to your God ; one of the children of the free-woman ; the former, however, cannot, as it is but a type and prefigurement, fulfilled, realized and properly understood only in Christ Jesus, because He Himself is ultimately the originator even of the type. He holds the keys to its proper meaning and understanding, being the author of it, and its fulfillment.
My deepest concern is of the human inclination to return to things familiar and known and restore past habits for the sake of mere tradition. There is danger that we become tempted to revert, whether explicitly or - equally as important - implicitly, that is in heart, desire or spirit. Many customs and traditions of my own family are snares to me, for example, as they are the fruit of what is a practical atheism. Doing them without believing or accepting them is always difficult, and still more difficult to justify outside the context of utter necessity or even outright force.
I therefore am unchanged in my opinion, and feel I must warn you not to permit yourself to be even tempted to make Esau’s most grievous mistake, wherein he exchanged his birthright for a simple meal.
Pax Christi,
Tim