You will not use the verb 'convenir à quelqu'un' to mean to suit in the way of a garment like a dress.
It would be 'aller bien' -
La robe va bien à ma sœur
Ça convient bien à ma sœur de venir demain = It suits my sister very well to come tomorrow
'Convenir bien' here means, it is acceptable or convenient to her and the preposition used is 'à'.
Hope this helps!
Think of "convenient" in English to give you a hint of the French meaning.
Ça vous convient de nous rendre visite demain? -- Does it suit you to visit us tomorrow. "Is it convenient for you to visit us tomorrow?"
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