The amount of luggage your carrying would impact your ability to wonder around looking for her.
Normally the crews will put the NYC passenger in towards the front. Keep the shorts next to the cafe car. The first coach is normally closed into Albany. The other coaches can get quite crowded.
Have your sister text you where she is would be best, but in Syracuse you will be told to head forward to find a seat for NYC. So it’s just finding her after you turn right upon entering.
Between Utica and NYC the river will be on your right side, small towns on your left.
A little extra detail the cafe car is the last one with 4-5 coaches forward to the engine. The conductor will open two doors one in front of the cafe car, and the other in the 2nd to last car. They may scan your ticket at the platform or direct you to turn right (as a NYC passenger) towards the front of the train. It can get quite crowded around the opening doors, but if your sister can just stand up and flag you as your walking in. It really shouldn’t be an issue.
Finding two seats together? Ask nicely to your fellow passengers, someone will be rude, but most will be happy to shift for you.
Enjoy your trip.