I'm a female and I have had a DNA test through Ancestry, and so has my niece (my brother's child). I have serious questions as I don't really understand how DNA is divided other than decreasing or increasing through the generations. Anyway, I have a first cousin 870 cM (male) on my father's side of the family who has a greater cM value than my niece 790 cM. I really don't know why. I know she received 50% of her DNA from her father (my brother) and 50% from her mother. Just as my cousin did. Can anyone explain why the numbers are less for her and more for him?
Here are two websites that I have used.
The percentage you inherit from your parents is an average of 50%, but can vary, there is randomness involved.
A niece average match is 1800 cM, 99% will be in the range of 1300 - 2300 cM. A match of 790 cM would be very improbable but not impossible.
More likely IMO, your brother is really your half-brother. More test results from other close family members could help confirm this theory. But, realize that you may uncover family secrets that some don't want to uncover.
Is there any other evidence that suggests that you or your brother have different parents? Look at your other results, are there any close matches that you can't explain? Do you have matches to both of your parents' families?
If your parents are correct, then it could be your brother who has an unknown parent. If you are on good terms with your niece, see if she wants to help you investigate. See if she has any close matches that she can't explain. This could lead you to the mystery parent.