My younger sister is deaf and my younger brother has never been on a plane before, so I find it pretty important to sit with them.
Your younger sister will probably qualify you for preboarding. Check with the gate agent.
Now here's my rule. NO seat saving. I take the seat. (I have had a woman try to get me moved. Had she nicely said she was holding the seat I might have moved but she had spread "items' out over THREE ROWS!!!!! So folks started MOVING her stuff. LOL! I wasn't the only one and she was ballastic! The SW FA actually started removing her stuff as she was throwing it on to other seats and informed her that if this continued she would be removed from the plane! She was furious that her family of about 12 had not been allowed to pre-board with her and her child. Her DH had preboarded too but he was in another row and did nothing to stop folks from sitting with him, which made her YELL at him LOL! I think he was trying to pretend he had never met her. Her family finally gets on. They had A passes but waited until the END to board because she was "saving" them seats. They wound up all over the plane and the SW attendants did NOTHING to help them get thier younger kids with adults. They had blown this one big time!) IMHO it's fine to save say a seat, but not whole rows.