If you want to say that your sister looks good in "pink clothes," the verb "to suit" expresses that. If you want to say that "pink clothes" are the right size for your sister, you'll use the verb "to fit."

You can use both with clothes. "Clothes fit my sister" means that clothes are the proper size and shape for your sister. To say this you'd need to say "These clothes" or "Those clothes" to specify which clothes; simply "Clothes" does not work particularly well. If you do use "These", "Some" or "Those" and other words like that, you can add in the word pink, "Some pink clothes fit my sister" or "These pink clothes fit my sister" makes sense, but suggests not all pink clothes do, and it is the clothes themselves that determine whether they fit or not, not the fact that they are pink (this is just an extra detail). "Pink clothes suit my sister" means "My sister looks good in pink clothes". In this case, you need the word "pink" (and/or any other adjective for example "loose-fitting" or "floral") as "Clothes suit my sister" sounds weird. In this case, the pink part does usually determine whether or not the clothes suit the sister or not, in addition to the clothes.

Bethany has given you a good explanation. I'll just stress that "suit" would probably not be used in this way in the US. It's a bit old fashioned. We'd say, "Pink clothes look good on my sister."