I became a Big Sister through Big Brothers / Big Sisters when I was single.
I also volunteered in some activities with kids.
The relationships lacked the permanence of having a child, niece/nephew or foster child, but I got a lot out of them, and learned much that helped me when I did become a parent.
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Mom to an amazing young adult AS son, plus an also amazing non-AS daughter. Most likely part of the "Broader Autism Phenotype" (some traits).