But this is all technical BS - an actual decision is made about how that child is treated & that's where the difference lies in being good or shitty, sorry 🤷🏼♀️
As an example -
My eldest DS is my husbands step son. DS now has a baby. DH is now a grandad & absolutely adores his grandson. Eldest is DH son & he loves him as such. He brought him up, loved him, advocated for him as his son.
FIL got remarried a few years ago. His new wife is step-mum technically but cos we're talking adults, she's not mum to DH & is referred to by her name. None of our 3 children are related to her by blood at all. She has no children of her own & is not remotely maternal. She's a strong, independent woman & I've got a lot of respect for her. She's improved the relationship between DH & FIL no end cos he was a bit selfish & distant prior. We see them a few times a year.
HOWEVER! Entirely of her own volition she now considers herself great grandma to a child who is not even blood to her husband. She looked fit to bust when she first met DGS, just smitten & immediately referred to herself as great grandma (which really took me by surprise). They've both stepped up to help eldest & his girlf out financially, we've just handed over a beautifully wrapped gift to DGS they sent him.
Sure, no one has to feel that way but that's the difference between good, ordinary, emotionally mature people & shitty elitist ones. The former just are that way, the latter choose to be.
OPs in-laws happen to be the latter & sadly for her, so does her husband.