Do I hAve to claim the funds for the whole year if I am The legal guardian of my sister?I only have received income for her for 3 months. Someone else claimed her?

My 14 year old sister lives with me full time since September. I am legal guardian have collected SS benefits for her those 3 months. Someone else claimed her on their taxes, do I have to claim the whole years SS benefits or just the 3 months.