Distinguishing niece from sister using shared percentage DNA

Question

On Ancestry who I thought was my niece my oldest sisters child shared 49% -56% dna how is that possible ?

I also have a first cousin who shares 26% yet his sister only shares 6% why is that

Answer 1

From your question I am unable to discern which individuals you are listing each of those percentage match values for. I am also not sure why you are expressing a range of 49%-56% rather than a single value.

However, at DNA Painter:

• a percentage of 49% equates to 3,646cM and suggests a Parent-Child relationship is 100% likely.
• a percentage of 56% equates to 4,166cM, does not suggest a relationship, but does say:

Assuming no pedigree collapse or endogamy, and that you're related in just one way, the furthest back you might need to go to find common ancestors for a match of 4166cM is Grandparent level. The connection may be closer.

• a percentage of 26% equates to 1,934cM and suggests several relationships may be possible.
• a percentage of 6% equates to 446cM and suggests many relationships may be possible.