Welcome AMJT: I'm so sorry about the situation with your sister. Sounds like a bad combination, to have both BPD and her level of OCD. How has she gotten away with pulling her daughter from school? Is someone home schooling her? I don't know where you live, but in the US, you can call Child Protective Services and make a complaint (perhaps an anonymous one). That may be the only road to change.
The link below leads to information about domestic violence and suicide prevention. In one section, you will find information about a Safety Plan. Check it out. You may want to share it with your parents, so that a plan could be developed on how to handle your sister. Perhaps they could get a therapist to assist.
SAFETYSAFETY FIRST- CLICK HERE A Safety Plan might need to include calling in the police, to deal with a suicide threat or domestic violence. Your sister could then be taken to a medical facility and evaluated (without her consent). That might be the only hope of her getting help. Some meds and therapy could benefit her. The future seem rather bleak for both her and her daughter, if she remains the way she is.
The link below can be helpful. The website has information for the family of suicidal people, as well as actual suicidal people. If you are in the US, calling the phone number can lead to gaining some support over the phone.
SUICIDE PREVENTIONwww.suicidepreventionlifeline.orgI'm thinking that some measures needs to be taken to protect your niece and her future.  :)o you think you might be able to help coach your parents with this? If nothing else, perhaps you could make an anonymous call to whatever child protective organization applies to where you live?