Answer:
die Mutter
der Onkel
der Sohn
die Schwester
Step-by-step explanation:
First, we see that all of the words you have to find are "attribute of the subject" (I hope it's the good word ^^' ) so you have yo use Nominativ, which means "der" and "die". Don't forget we have to agree the determiner with the gender of the name.
die Mutter : it's his mother, so it's feminine
der Onkel : it's her oncle, so it's masculine
der Sohn : it's his son, so it's masculine
der Bruder : it's her brother, so it's masculine
die Schwester : it's their sister, so it's feminine