You are very nice to do such a thing for your brother! As climbermom said, be sure to get all of your paperwork in order - most airlines have information on their websites about exactly what the notarized letter must say.

In terms of advice for the flight, I just flew PIT-ORD-NRT with my then 4.5-year-old and 21-month-old a few months ago, so here is my long, long post based on that.

Even though it might seem counterintuitive, get as direct a flight as you can (stopovers mean running-around time, but they are also very tiring and disruptive) - that will allow you to sort of "nest" yourselves into the flight, and means that the kids will probably sleep for a good part of it.

I second the idea that you should get a seat for the younger child, though you may not want to bring a car seat. The upside of a car seat is that it is a place where you can firmly secure the child, and the child might sleep in it. However, the negatives are that the car seat will make it easy for the child to kick the seat in front of her, you won't have as much space to spread out, and you will have to haul it through security, the airport, etc., when you already have your hands full.

Be sure to use your carry-ons to take plenty of entertainment for the kids. There are suggestions in the sticky that are helpful. In particular, I would recommend books and crayons for the older child, and many, many small toys for the younger child. The dollar store can be a good source of these, and you don't have to worry about losing any of them. You should also think about bringing along a "lovey" (blanket and/or stuffed animal) for each child to help them relax and go to sleep. If you have a portable DVD player, stock up on kids' DVDs from the library, and buy a $3 earphone splitter jack from RadioShack so that both kids can listen. I don't let my kids watch more than 1 hour/week of TV when they are at home, but I'm all for sedating them with the boob tube when we're on a plane! If your nieces do watch TV, consider eliminating all TV from their lives for a week before the flight, so as to increase their interest in the DVD player.

Dress yourself and the kids in layers, so that they can be comfortable both on the ground, and as the plane gets colder mid-flight. You can use your layers (sweater, etc.) as secondary blankets to tuck in the kids. Bring twice as many diapers and wipes as you think you will need, and a spare outfit for each kid in case of potty accidents or airsickness (have the 4-yr-old go to the bathroom right before you board). Also, bring an empty water bottle (or 2) to fill up before getting on the plane, and a disposable sippy cup for the younger child, so that both girls can take a drink as the airplane ascends/descends and the pressure changes. Although you will be fed on the plane, you should also bring some snacks (crackers, raisins, etc.) for the long gaps in meal service.

If it is at all possible, consider flying on a foreign carrier (ANA, JAL) instead of a domestic one (United, Delta). On average (not always, of course), the flight attendants on ANA and JAL seem to be much, much nicer than on US domestic carriers. ANA and JAL also stock little toys that they give to kids on their flights.

Once you land in NRT, the security lines can be long. However, the line wardens very often seem to pull families with small children out of line, and send them over to a much shorter one. It doesn't hurt to ask.

Best of luck! If you are prepared, it really won't be so bad, and you are doing a wonderful thing for your nieces and their parents. ^