I have no problem giving money or vouchers - I love going to weddings, I never expect an invitation, but if one comes I feel privileged to be asked to share their day and really it's just a way of helping to contribute to the cost.
My view is perhaps coloured by our years in Hongkong - we were invited to several Chinese weddings. The Chinese have no sentimentality about gifts, and the standard (expected) practice was to give the happy couple a small red envelope containing 'lucky money' - HK$100 to be precise, which was then worth about £10, so the expected amount is probably rather more by now.
In answer to the OP then, if you go to the wedding, of course you must give a gift, the value of the gift being dependent on your own means and how close you are to your nephew/niece.