HI,
So Messages 8 in Yosemite.
Using an AIM account.
If you have not already use the item in the Messages > Preferences > General Section to ungroup the Buddy Lists.
With separate Buddy Lists use the Window Menu to display the AIM one (if they are not already)
With this Buddy List "Front" (the Window with the active traffic light blobs top left) go to the View Menu and Show Off Line Buddies.
Does your sister's AIM Screen Name now show up ?
IF so we have to work out why this might be.
GO to the Preferences > Accounts > AIM Account and the Privacy tab
Are you using any of the restricted Allow options (Buddy List only or Specific People) or any of the Block items ?
If you have set Allow Specific people is she in the list ?
If you are using Block Specific People have you added her ?
You may need to get her to check these setting at her end as well as if she Blocks (or does not allow) you then she will appear Off Line.
Also if she has changed her Status to Invisible (either at the top of a Buddy List or the Messages Menu itself) then you will not be able to see her as On line.
7:45 pm Sunday; February 8, 2015
iMac 2.5Ghz i5 2011 (Mavericks 10.9)
G4/1GhzDual MDD (Leopard 10.5.8)
MacBookPro 2Gb (Snow Leopard 10.6.8)
Mac OS X (10.6.8),
Couple of iPhones and an iPad