I shot this a few months back in about ten minutes, including getting the single strobe and shoot through brolly set up, when they asked I'd take their picture one day.
Well, they're fortunate! I like the real happiness. You niece is so cute and is really having fun. She a light bulb herself! Her mother seems a happy lady. The two of them do go well together. The child being held and leaning in to her motehr is is a nice touch. I have a niggle or two. Your brother is rather tall! He certainly seems a warm fellow. I like the picture and I'm sure they will treasure it.
It's interesting to me that each of the 3 faces and poses are differently drawn. Of course the child's face is perfectly smooth and the others are more mature, but it's more than that. The girl is really enjoying the moment, even relating to you. Her mother is intent on making the composition one of closeness and your brother stands behind them, glowingly proud that he's there. All are different.
The other thing that's worthy, is the color of the picture. B&W, is of course not either. The portrait has a touch of wood bark or siennna, just enough so it's not glaring. Is that the Rodinol or what?
Asher