Hi Dot,
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!
