This is the 13054th most frequent Korean word.
동생이
"동생이" means "younger sibling (subject marker attached)" in English.
In this sentence, '동생이랑' means 'with younger sibling', where '동생이' is the base form indicating 'younger sibling' and the particle '랑' indicates accompaniment.
나는 동생이랑 같이 학교에 갔어요.
I went to school together with my younger sibling.
Here, '동생이' identifies the subject of the sentence (my younger sibling), as the particle '이' helps mark '동생' as the subject.
동생이 숙제를 다 끝냈어요.
My younger sibling finished all their homework.
In this use, '동생이' serves as the subject within a subordinate clause ('더 좋아한다고'), specifying whom the mom favors.