"I am going to visit my sister". (Present continuous tense)

妹に会いに行くつもりです verb + "に" + verb. Japanese often use this grammer. It is difficult to explain but, it expresses a series of movements. for example, 会いに行く　会う+行く 食べに行く　食べる+行く 遊びに行く　遊ぶ+行く In the same way, 〜に+verb is seen in " adjective + verb. here are some example. 好きになる　好き+なる 上手になる　上手（じょうず）+　なる it may confuse you, but please try to search more if you couldnt understand.

妹に会うつもりです

While two verbs beside using ”に”? 会いに行きます？

妹に会いに行きます