I’m fostering my nieces and I’m pissed

Backwards family.

So my husband (23) and I (23) have recently begun fostering our nieces ages 15 and 4. Home life was bad, horrible living situation, feces, urine everywhere, 4 kids 4dogs 2adults 3 rooms in a house. They eldest has horrible health, physical and mental, as well as no schooling. She’s around a 2nd grade level when it comes to school subjects.

How we got them- I got a call from my sister in law that her kids were being removed and she essentially guilt tripped and threatened me into wanting to take her children in. (The threat was my husband never speaking to his brother again) I explained we could take the girls but didn’t have room for the boys. She agreed. Ever since we agreed she has been giving us hell about taking care of them “properly” essentially fighting every medical decision, school decision, and mental health decisions. Example- she was screaming and fighting with us about having the eldest tonsils removed (when we got her she barely had room to breath) saying it’s against her religion…(she doesn’t have a stable religion, she pulls certain things she likes and agrees with from ever religion).

Recently the eldest two 15f and 11m have been having a lot of mental health issues and aggression. Where her 11 year old brother prefers physical violence and manipulation she prefers her mothers choice of mental and psychological manipulation. She has found that “seizures” get her what she wants so she takes them whenever she is uncomfortable. (Not saying this lightly I have seizures and know what to look for plus we have had her examined and hospitalized mutiple times with no seizure activity including during her seizure only thing elevated was heart rate.) She has no distinction of reality compared to fiction and has said how she sees a male animal cartoon character (she insists they are dating) and how if we do anything he will hurt us.

The younger girl is 4 and has the biggest personality, I honestly love her but she also has major anxiety and night terrors and some of the things she has told me about their own home is insane and makes me so very angry. Oh also the 4 year old isn’t related to my sister in law at all she had temporary guardianship that expired and she fled the state with the kiddo so that’s its own separate story.

We have had custody of the girls since April and I’m just needing parental advice or something that says I’m not crazy while dealing with these things. It’s also kinda me venting. I just feel crazy and I want to know if anyone has ever dealt with something like this. Any parenting tips are much appreciated when it comes to the 15 year old.