跟 joins a co-actor who shares the verb; 向 aims the verb one-way at a target who receives it. 跟他學 = study alongside him; 向他學 = take him as the model to learn from.
verb before person: 我去跟姐姐圖書館 → 我跟姐姐去圖書館
treating 跟 as a verb: 我跟姐姐。(I'm with my sister — incomplete) → 我跟姐姐去圖書館。
stranding 跟 like English 'with you': 我去跟你 → 我跟你去
English 'with' trails the verb (go with me); 跟 sits before the person and before the verb (跟我去), so learners place it too late.