I am meeting my sister in Brussels on Thursday morning. I am flying from JFK on Delta and arrive around 9:30am. She is flying from London Heathrow on BA and arrives at 8:55am.

Does anyone know a spot for us to meet easily? I don't think are phones will be working, so that isn't an option. Should we just meet in the hotel? THANKS!!!

You could either meet at the exit of the baggage claim & customs or just wait in the Wing Tips restaurant.
If you and your sister will have to take the train to downtown Brussels, you can also meet downstairs at the airport's train station ticket windows(there is only one way to enter the underground station from the terminal).

There should also be an official "meeting point" like in all other airports, but I am usually too stupid to spot those ones.

Thanks. Do you think there is only one baggage claim/customs area?

Also, the airport's website says there are three wingtips. Do you know which one would be where we would be exiting?

These are some good ideas but just in case there's a mix-up, are you sure your phones won't work? Almost any cell phone made in the last several years will work in Europe. Your provider should be able to tell you if you have coverage.

meet at the elevator to go down to the train station

I also heard that there is a Java City in the public area of arrivals. We could meet there after going thru customs and getting our luggage, right? Thoughts....

I really appreciate your help in finding my sister.

I have given you a reply to this question.

I am sure that as an adult you can comunicate with your sister and make some arrangement like - at the information desk or at the BA ticket sales desk.

There is only one way to get from the baggage carrousels/customs into the Arrivals area, regardless of your flight. People all come out of the same set of doors. Why not just have your sister wait for you there?
Alternatively, go with Lawchick's suggestion and meet at the coffee bar.
The arrivals area is pretty small anyway (much smaller than the departure area), so it shouldn't be too hard to find each other.