I don't think I am missing the point, though.
You are comparing in-laws with blood relatives as if that's a meaningful comparison - but it isn't. In-laws are people who are related to you by accident of marriage, they aren't people that you've actively chosen to have in your life, and they haven't actively chosen to be in yours.
A much more meaningful comparison would be friends vs siblings, and for most people I know, friends are a more important source of support.
When my friend plotted to leave her abusive husband, she turned to me and two other friends to help her get away safely. Her three siblings were aware off her plans but they weren't involved.
When another friend needed help advocating for her disabled daughter's education, it was me that helped her write the letters and prepare for all the meetings etc. She didn't even bother talking to her sister about it.
When a third friend needed to look after her dying mother, it was me that had her kids to stay with me for a few days, and not their uncles and aunts.
And since my own mum died and I have had to take on caring duties for my elderly dad, it has been my friends who have stepped up to ensure that I get some respite from time to time. My dsis makes all the right noises but she is too busy to actually do anything.
I understand that some people struggle to make friends, and I can see why siblings would perhaps be more important if you don't have good friends in your life, but I really don't think that's the experience of most people.