When you say you are a quarter Japanese, that’s by lineage but DNA can be very different. For example you could be 25 percent Japanese but your sister is only 10 since she got a different mix of DNA so to speak.
For example it’s possible You have distant African-American grandfather (great, great...etc) and you have no African-American DNA whatsoever, while you brother is 4% African American.
These numbers are just examples.