There is a huge difference between naming someone a person of interest and a suspect. There are reasons why LE names someone a POI rather than a suspect, but I forget specifically what they are.
Also, I can understand not meeting a half sibling, or stp sibling, but not having any knowledge of their existence until recently? KS is older than BM. I can't imagine a situation where BM's mom and dad were together and she had no clue the man she was having a child with had a child with another woman a few years older than the child they were expecting or had.
Is KS a half or step sibling, do we know?
ETA: Sorry nurse, I posted this before seeing your post above. In many cases persons are named a person of interest long before being named a suspect. We've discussed the difference in other cases. I will await word from you if we can discuss this case in the same manner, understanding the difference between LE naming a POI and naming a Suspect. tia!