Sydd versus sy'n

Is there any difference between sydd and sy’n in usage,
Dw i’n nabod dyn sy’n/sydd gwybod dy chwaer

There is no difference, but sy’ is just the shortened form of sydd. Do keep in mind that if the linking yn is needed, then you need it in both cases:
Dw i’n nabod dyn sy’n / sydd yn nabod dy chwaer.
Compare with (for example):
Dyna’r unig beth sy’ / sydd angen (no yn in either case)

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That brilliant, thank you for your answer