Yes, i know. I assumed it is returning None because for example age(70, 4.666666666666667): in line 4 "julie / y" results in 14.999999999999998 instead of the expected 15. Thus there is no match to "== brother" in the same line and therefore the if-statement fails and there's nothing to return. Am i mistaken?

That's not why your code fails. Your code is returning None for some inputs.

Tests failing to lack of rounding (f.e. 14.999999999999998 instead of 15). Please help!

My code might be the ugliest solution in the world, but it works when put into my console. However some test cases do fail. Any advice?

Yes, i agree on this one.

Maybe looking up default arguments in python helps if you are struggling like i was.