Sexual abuse?

I do hope you can help me.
My niece who is 3 years old has recently told my sister (my nieces mother) that my sisters boyfriend has "touched" her in "private" areas and he has forced her to watched pornographic movies with him.
He does treat my niece badly, showing more affection for his blood daughter and not my niece. My nieces baby sister is 2 yrs old and does recieve 99% of the attention, attention that my niece did once have.
Now that everyone has talked to my niece and basically called her a lier, she tells me it didnt happen now.
A few months before she even stated that this assault took place, she was acting very differant. She would not let anyone leave her alone at all, not for a moment, she was very very clingy, she stated how badly she hated her step dad and would "ball" when she knew she had to go home.
The night she told my sister, my sister and her boyfriend did get in a big fight, which was verbal and physical.
My niece has mention sexual abuse in the past , but never ever like this. She was also caught asking my 6 year old and 10 year old boys if they "knew" how to have sex.
I am so afraid that she crawled into a hole beacuse nobody believed her......how can I really tell...Because I call children services all the time, I have her with me most of the time and I am constantly in tears!!!
Please Help!!

[AnnieBrooke]

Why is the child still in that living situation?  Why is your sister in a relationship where she is subjected to physical and verbal fights and possibly letting her daughter in for sexual abuse?  

[margypops]

those were also my thoughts this child should not be exposed to this it may be time to seek outside help it does sound as if she was abused... in my opinion children of 3 year old do not make up stories like this so believe her and get help ..

[Athena_M]

Call child protective services.  This girl should not spend anytime with the boyfriend at all.  I really don't think she made this up. It's good that she has you in her life, maybe she can stay with you.  

[mammo]

Three year olds do not have the capabiltity to make this up!  Now she is withdrawn due to being treated as if she did something wrong, which is so unfair.  Call 1-800-for-kids and get her out of that situation.