Love of ones child is ingrained in nature by GD. It's free. Doesn't take any effort or sacrifice.
A spouse on the other hand....
exactly. I believe theres an Ishbitzer torah on Avraham asking Sarah to say for me that you are my sister: we are starting a long and difficult journey and if we are just normal partners/couple for convenience we could never get through without grating on each other and ending it. but when you are family, you truly feel like u are in it together and could never give up the blood bond you have. So Avraham asks sarah to bring their love up to the level of blood relative, where there is an inherent bond that cant be broken.