Meet sister LHR, then Tube to St. Pancras

Taking the Tube from LHR to St. Pancras is a straight shot, which takes 55 mins.

You can board the tube at the station in T5:

http://www.heathrow.com/transport-and-directions/underground

Since you arrive first, I suggest you go to T5 to meet her. Proceed through passport control to "landslide" in T3, then use the Heathrow Express ( for free) to transfer to T5 ( make sure to get it in the correct direction!). You can then go up to the arrivals hall in T5 and meet her as she exits the customs " green line" door ( there is only one). There is a sort of barrier here that people waiting to meet incoming passengers line up along to spot their quarry--- it is easy to do. You won't miss her.

Just remember it can take an hour or more from the time her plane lands for her to get through passport control and walk to the customs exit door.

We did this exact thing when meeting up with my sister and her husband 2 years ago. We did have communication by cell phones ( text) in case of a glitch, but all went smoothly. They were standing at the barrier, holding a sign with our names as if they were limo drivers sent to pick us up. Made for a nice joke. We then rode the Heathrow Express into London together, but the same plan would work for taking the Tube.

Just be sure you both have a clear understanding of the plan, and a back-up plan. And hopefully UK-enabled cell phones.

Donna, If your sister wasn't "directionally challenged", I would suggest each making your own way to St Pancras via the tube (Piccadilly Line). A lot more convivial place than Heathrow. Meet up there either at the Champagne bar, or by the statue under the clock.
Photo: http://citytransport.info/Digi/P1270287a.jpg

If you don't think she can manage this, just you go to T5 (train as already described), and meet here there.

And make sure you both have working mobile phones if it all goes pear-shaped.

Once she comes out of the customs area and into the main terminal, she can either go right or left (there is a metal "fence" directly in front.) Have her go right. If you are meeting her, this is where you should be.

From there follow the signs to the Underground. If you need to buy tickets, there is a booth as well as machines just before going through the doors marked "Underground."

Take either the elevators or escalators down to the Underground. (There will be two sets--one for the Underground and one for the Heathrow Express. Make sure she takes the one for the Underground.)

Once at the right level, she will find the turnstiles she needs to get access to the train. Just pass your Oyster Card over the reader and the gates will open. There is no worry about which train to take as there is only one line and Terminal 5 is the end of the line. It only goes in one direction. If there is no train, there will be an electronic board telling you when the next one will arrive. There is usually someone there to offer help if needed.

Get on the train and take it to St Pancras. No need to change.

Thank you all so much for the detailed replies. I've forwarded them to my sister. I think she/we can do this thanks to you all.