Dear StarPonysMama,
I'm a psychology student and it seems to me that your stepdaughter should see a mental health care professional specialized in children. It's not normal for a child not want to tell how she hurt a part of her body. If she hurt her colarbone on the trampoline, she would've known and told you right away.
At some point, you expressed concern about "a little emotional abuse" rather than physical abuse. There's no such a thing as a LITTLE emotional abuse. There is only abuse. It's not up to one person to judge another's pain. What is little for me might me a lot for you, and vice versa.
I was physically and emotionally abused. I know what it's like to be hurt. Trust me, it hurts forever. If your stepdaughter (or any other child) is being emotionally abused, it is just as serious and painful as if she is being physically abused.
You promptly noticed how her mother and older brother were being nasty, calling her a whiny baby when she didn't want to go with them. If any kid at the playground treated her like that, it would have been considered bullying, teasing, and provoking. I can assure you that kind of criticism is way worse and more hurtful coming from a parent or an older sibling, who are some of the people children ideally rely on for support. Her mother is stupid and mean, to say the least.
Please, take good care of your stepchildren's emotional wellbeing. They will be forever grateful to you.
Best regards,
Nut