似る and 似合う: resemblance and looking good together
Distinguish resemblance from clothing suiting a person, and compare the state 似ている with the evaluation expressed by 似合う.
Who someone resembles
弟は父に似ています。 says that the younger brother resembles the father. 似る, read にる, uses に for the comparison target; current resemblance is expressed with 似ています.
この二つの漢字は形が似ています。 says that the shapes of two kanji are similar. Similar shapes do not establish identical readings or meanings.
Whether clothes suit a person
その帽子はあなたによく似合います。 means that the hat suits you well. 似合う, read にあう, does not mean that the hat resembles the person.
青いシャツが似合っていますね。 comments that the blue shirt looks good on someone. It evaluates the appearance, not merely whether the shirt is the correct size.
Distinguish both form and evaluation
似る becomes 似て and 似ない; 似合う becomes 似合って and 似合わない. Check the verb ending rather than blending them into a form such as 似合いて.
母の服に似た服を買いましたが、私にはあまり似合いませんでした。 means that the speaker bought clothes similar to their mother's, but they did not suit the speaker very well. Resemblance and suitability are separate judgments.
Does その帽子はあなたに似合います mean that the hat resembles the person?
Show the answer and why
No. It means that the hat suits the person. Resemblance is expressed with 似ている, with its comparison target identified separately.