People who find that 'it all comes naturally' are probably the worst teachers of a subject!
For example, I did well at maths at school (by asking lots of questions) until I encountered an A-level teacher who (while gifted at the subject himself) simply couldn't understand how somebody couldn't see things the way he did. However, having to struggle through A-level maths myself almost certainly helped me to teach it in later years, as I could understand the dificulties that my pupils were experiencing.
I also know that there are some people (most definitely including me!) who simply can't capture a picture in their heads. For example, if I looked at my mother and then turned away from her I couldn't see a picture of her in my mind at all. Unless I'd deliberately made mental notes about her height, build and, say, the colour of her eyes, I wouldn't have been able to tell you whether she was short, tall, fat or thin, yet alone anything about her face.
So I can understand both your own difficulties and, possibly also, those of your niece.
However I do know that the BBC website has lots of useful pages (including videos) about learning to draw. A web search for 'BBC learn to draw' seems to find lots, with 'BBC drawing' finding further helpful pages. Perhaps similar web searches might help you?
(You could also try searching on Youtube for suitable tutorials).