There will be a total of (1) Brian + (1) Brian's wife + (2) two kids + (2) Brian's parents + (2) Brian's in-laws = 1+1+2+2+2= <<1+1+2+2+2=8>>8 people eating
If 1 person eats 2 serving of lasagna then 8 people will eat 8 * 2 = <<8*2=16>>16 servings
If each serving requires 1/2 cup of milk then 16 servings require 16 * 1/2 cups = <<16*1/2=8>>8 cups of milk
If one cup of milk equals 250 ml, then 8 cups of milk equal 8 * 250 = <<8*250=2000>>2000 ml of milk
1L carton of milk equals 1000ml of milk, therefore Brian needs 2000/1000 = <<2000/1000=2>>2 cartons of 1L milk
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