형 is only correct if you say brother not older brother but 동생 is only correct if you put younger sibling not sibling... A little bit of consistency would be good...
동생 doesn't necessarily mean "younger sibling". It can refer to the very many different types of sibling situations and siblings in itself. It is also used to pronounce a 'sibling'. For example, the phrase "my sibling" can be translated to "내 동생". Other terms are used to clarify "동생". (i.e. to single out; such examples are titles like "older sibling" and "younger sibling")