Is it a sin for a Catholic to have a Seder?

No. Jesus had a Seder at the last Supper

Roman Catholic AnswerThat depends. There are over six hundred commandments in the Torah. They are divided into three categories by St. Thomas Aquinas: Moral, Legislative, and Foreshadowing of Christ (not the category names he used!).

The moral laws are universal and thus are good for all time, in other Words, we are bound by the morals laws, like the "Ten Commandments".

The Legislative laws may or may not be valid depending on the civil laws of the nation you live in.

The laws which foreshadowed Christ ARE a sin, if you are observing them because you believe that Christ will come, in other Words, if you don't believe that he already came in the person of Jesus two thousand years ago. Thus we are no longer to obey the rituals such as the dietary laws and the ceremonies which were fulfilled in Christ. The Seder has its fulfillment in the Last Supper and the Mass. To fully observe a Seder would be to acknowledge that you do not believe that Jesus fulfilled this at the Last Supper and established the Eucharist, thus it would be a sin.

For a full treatment, please see St. Thomas Aquinas Summa Theologiae.