And this lesson is mentioned as "explain this". Whilst the lesson really explains usage of le / la, in this case it does not make any sense because I previous lessons did not have either of those four words, so a student just doesn't know what those words mean.
It doesn't matter what the words mean. The only point is that le and la become l' before a vowel and not before a consonant. So le écharpe should be l'écharpe and l'souris should be la souris.
The correct articles are la soeur, l'oncle, l'écharpe et le souris.
You don't need to know the gender of 'oncle' or 'écharpe'. They start with a vowel, therefore, use l'.
Soeur is feminine; souris is masculine.
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