dwl,
Congratulations on your nephew's wedding.
The dresses for the engagement ceremony sound lovely. I'm not sure what a longe suit is, but if it's an ordinary suit, I think that's lovely, too.
At the wedding banquette, you are expected to wear the same thing you wore at the wedding ceremony.
As for the monetary gift, typically expected amounts are anything between 30000 and 100000 yen per person, except for 40000 yen and 90000 yen which are obviously unlucky numbers. A typical monetary gift sum for aunts, uncles, non-student siblings and grandparents would all be 50000 yen per person. If the host(ess) has insisted on paying for your accomodation, flight tickets or kimonos, you might want to add some extra amount on your monetary gift as a gesture. The mother, of course, will not be giving the monetary gift, as she is hosting the wedding as a parent (but the groom is free to ask her for financial support).
By the way, seeing all these English questions on monetary gifts, I'm surprized there aren't English websites that explain them in detail, but here is an example in Japanese language.
http://allabout.co.jp/family/ceremony/closeup/CU20071010A/Please scroll down to the middle of the page where you see 3 charts based on questionnaire.
The top chart shows average amounts. On the left row, from the top it says, siblings, aunts/uncles, cousins, other relatives, bosses and so on. The middle row shows the sums that most people answered. The right-hand row shows the average sum.
The lower charts shows average sums depending on the region. On the pink blocks, from the left it says Hokkaido, Tohoku, Northern Kanto, Tokyo, Southern Kanto, Chubu, Kinki, Chugoku, Shikoku and Kyushu.
The idea is that the couple (sometimes with financial support from parents) will be entertaining the guests with expensive meals and gifts which typically sums up to about 30000 yen per person, and they'd often plan to cover the expenses with the monetary gifts.
Typically you are expected to put neat bills in the number of envelopes according to the number of invitation cards you recieved. For example, if you recieved 1 card for the aunt/uncle couple, then the aunt/uncle couple will give 1 joint envelope. But typically you will be handing the envelope directly to your nephew in advance to the wedding day, so I don't think there's anything to be nervous about.
Also I would say, although some may disagree, that if anyone is giving an expensive item as his/her wedding gift, that person can deduct the cost of the item from the monetary gift. The wedding gifts, be it an item or cash, should be handed to the bridegroom in advance to the wedding day.
Indivisual tolietries and good quality chocolates sound very nice as greeting gifts. But if you're worried about having "other family members there," you may want to pack one big gift for "the whole family." Otherwise, I suppose you can still give indivisual gifts but keep some extra ones in your purse just in case. Either way, don't be surprised if they don't open them in front of you, because that's the traditional way, although in modern days many would open the gifts immediately.
Have fun and don't be nervous!
