In terms of trust it may be true because a mother would never betray a child and their feelings are also much more sensitive to the people we leave behind. But the question is what if they have passed away first and do you have another way to store the seed phrase as a step that can be used when we are gone. I believe mothers are the most trustworthy people but other possible scenarios that may occur also need to be prepared for.
If that was the scenario it means you need to share the seed phrase with your child, because if you share with your wife or little brother/sister, it would be pointless because they're live in the same generation, they're likely to die earlier than our child, hence pass it to the next generation.
probably the only time that the latter would matter is if you've got a toxic and vile family relations that has gave you nothing but trauma, that's the only time that I think closest friends would matter when leaving something valuable is being talked about.
Only if you're single, people who got toxic family can still build a small family with partner and child.
It's completely hard to believe someone would gave his wealth to his friend, I only know there were few cases someone gave their wealth to housemaid.