How to hold salmon after cooking for travel to family dinner?
Most folks rely on smoked or cold salmon when travelling. It's a special treat getting hot salmon brought in. 

Cooking it right before you leave and keeping it warm will be the challenge. You might look at pizza delivery carry bags with the foil interior or create your own foil wrapped fish, wrapped in old towels and placed in a covered box with heated foil wrapped bricks (wrap a clean brick with foil and heat in the oven).

If you reheat the fish at your family's house be sure to follow these tips: use an oven (never microwave), reheat at a lower temp, cover the fish to keep it from drying out. https://www.thekitchn.com/5-mistakes-to-avoid-when-reheating-salmon-230418