An alternative to using a potato masher is to use a blender on pulse. Be sure not to blend them to a puree though.
Mash up the grapes using a spoon or a potato masher if they start clumping up or retain their chunkiness.
An alternative to using the sieve is using a cheesecloth. Put the cheesecloth over the pot and strain the mixture through (you might need to fold the cloth over twice). If you have a food mill, this would work beautifully.
