Sounding all good so far with councelling and police involved! So the family situation is the main issue now (if only I listened better!). And sorry for making assumptions about your sister marrying her deceased h brother...that'll teach me to jump to conclusions! Must have been a hard and traumatic situation. Very strange for your neice tho .
Families often cover up, and stopping your niece going out with friends is classic behaviour. If she went out she would see how other more 'average' families interacted. When I had a 'realisation' about my family at 16 (emotional abuse) I was told it was teenage hormones...at 16
The family think everything would be alright if you just admit your neice is lying because if you don't talk about it and cover up, the situation doesn't exist. Apperances count, because if this gets out...well they would not be normal would they? Try this book 'how to divorce a parent' by beverly engel. Its not totally about divorce, it includes separation and reconciliation too. Divorce is seen as the final thing to do with a family who you cannot be around anymore. I found this book helpful for me as it details the different types of family, all the different types of abuse under even emotional abuse. It also has a specialied section on sexual abuse. It helps to read it and read that what you experienced IS abuse and it also goes thro the responses you can expect from abusive parents/relatives when you confront the abuse. And ignoring it is one of them!
There is also a looooonnnnnggggg thread called 'My mother has cut me out of her life' on here. It may help your niece to read it. We have all been abused in some way (emotional/physical/sexual) by our mothers/dads/stepdads/siblings/other family and in some cases it details the stories, our families response to our coming out with the truth...they generally have a bad response...like your neices family. Some get very abusive, some just shut down all contact...all deny anything ever happened...may give your neice some strength to know she's not alone in her abuse and her mothers/gp desertion of her.
Anyway congrats on your neice doing her GCSE's, hope she gets what she wants from them. And you and your sister are still brilliant for standing up for what is right!