I would have 2 pieces of paper 1 for the things you yourself have actually witnessed and paper #2 for all the things told to you by a 2nd party. And keep in mind that that 2nd party is not YET willing to step up for her own niece’s sake out of some kind of fear of not being able to see her. And the niece is not YET willing to take a stand against her own mother.
I’m not saying NOT to make a phone call to CPS but when you do have the facts in hand. Discuss with them your concerns about the things “told” to you and the fears involved by the family in not stepping forward themselves.