Hello Maria,
Your relative is actually correct in this case. The spiciest part of a chili pepper is indeed the white inner membrane, which is botanically known as the placenta or pith.
Capsaicin, the compound responsible for the spicy heat, is synthesized and stored almost entirely within the glands located in this spongy white tissue.
The seeds themselves do not actually produce any capsaicin. The reason seeds are so widely believed to be spicy is simply due to their location. Because they are attached directly to the inner membrane, their outer surfaces become coated in the capsaicin-rich oils secreted by the placenta. If you were to thoroughly wash a seed to remove those surface oils, it would have virtually no heat at all.
If you want to control the spice level of your marinade without losing the fresh pepper flavor, you should slice the pepper lengthwise and use a spoon or butter knife to carefully scrape out the white inner membrane and the attached seeds. Removing the ribs takes away the vast majority of the capsaicin, leaving you with the much milder outer flesh.
I hope this helps you prepare a delicious marinade for your family grill this weekend!