"My own brother" is "Ο ίδιος μου ο αδερφός..." The word order changes. You can find the precise translation in the Drop Down Hints.
Find more help here.
TIPS TO HELP YOU LEARN + HOW TO REPORT A PROBLEM
And check out the Greek Forum here with more links.
Please read the comment above yours.
To say: "my brother ate the cake by himself" you would say ""ο αδερφός μου έφαγε το κέικ μόνος του""
Her we mean...Here it means..."My brother not someone else ate the cake." a slight difference in both languages. But they are different.