There is no excuse for a treif (non-kosher) seder. None.
Shalom bayis? (Peace in the home?)
There is no excuse for a treif seder.
Someone who regularly attends one or both non-kosher seders with his or her parents has several options to stay kosher for the seder meals.
1. Offer to go shopping with the chef (Mom or Dad). Armed with the Passover shopping list, hit the Passover aisle and use kosher-for-Passover ingredients and food. Reserve or pick up the kosher chicken or brisket from the live deli or kosher butcher.
2. If that doesn't work, declare your intention to have a completely separate kosher meal, which you will prepare yourself. I admit transporting all the food to the parents' house may prove difficult, and the kosher consumer may be fighting for counter and oven space in the kitchen, which isn't kaschered anyway.
3. Send regrets and attend the seders at friends whose seders are strictly kosher for Passover. Explain your commitment to keeping kosher for Passover. Suggest Sunday, April 4--the fourth day of Chol Ha'Moed*--as an alternative family visit day, when you can visit at home or do an activity around town. (Remember to pack a kosher-for-Passover lunch or snack for yourself.)
So many people spend hours cleaning their kitchens and preparing the seder meals. They want to show off their hard work by hosting many guests, including guests whose own kitchens are not kosher, and guests experiencing a kosher seder for the first time. Don't disappoint them! Take advantage of this opportunity to enjoy a kosher seder the way it was meant to be.
*Chol Ha'Moed refers to the intermediate days of Passover and Sukkos.