Women's jewellery, when they are within reasonable limits, are exempt from zakah. That is because they use the jewellery and they do not hold it as savings. However, a woman must pay zakah for her jewellery if she considers it a method of saving. However, if you give that jewellery to your sisters, without waiting for their marriages to take place, the situation becomes different. It becomes part of their property.
Zakah liability is applicable to each man and woman separately.
Therefore, each of your sisters is treated on her own. Each of them has to look after her own zakah. If you pass on the jewellery to them, they need not pay zakah for it, as long as it is within what is considered to be reasonable for women in their social status. Having said that, I must explain that when you give this jewellery to your sisters, the transaction must be a serious one. They must know and you intend that the jewellery becomes their own property which they use as they think fit.