Is it stepdaughter or daughter-in-law?
Both of them correct.
My stepdaughter is running whenever the weather is beautiful. was rejected.
For a repeated action, I think you should use the simple present "runs".
Shouldn't quand be used here instead of lorsque? In the explanation, quand means "whenever" (chaque fois que) and lorsque is only used for one particular instance. Or are they apparently interchangeable?
In many cases, "quand" and "lorsque" are interchangeable.
Both can refer to a repeated event in one context and both for a particular instance in another context.
Why "my daughter-in-law runs whenever is nice weather" is wrong??
My daughter in law runs during nice weather = unacceptable.