Also sorry for taking this off topic but I thought I might answer the question once and for all.
Whenever you buy a computer program you are also purchasing a licence to use said program. For the majority of programs that are purchased the licence only covers the installation of the program on one computer only. So that means you are not
allowed to install the program on more than one pc, and that also includes a personal laptop (so you are only allowed to install it on either your desktop or your laptop, not both). For some programs you can buy more than one licence. So, for an example, at my work I do a lot of work on ArcGIS, and the program is installed on my work pc. However we have a limited number of licences installed on my office work server (I believe it's four). So if more than four people are using ArcGIS and I want to use it I can't, even though the program is installed on my pc. Obviously work makes sure that there are enough licences for us all to work on. Another common example is Microsoft Office. Legally you are only allowed to install Office on one pc. So in a household, if there is more than one pc, you have to buy a copy for each pc. This is explicitly stated in Microsoft Office's End User Agreement.
Now for the sims, I had a quick glance through EA's Terms of Service (and I do say quick as I'm supposed to be working
) and it states that you are not allowed to "distribute, publicly perform or display, lease, sell, transmit, transfer, publish, edit, copy, create derivative works from, rent, sub-license, distribute, decompile, disassemble, reverse engineer or otherwise make unauthorized use of Content or Entitlements ". So unfortunately momofmany borrowing the base game from her sister was illegal, doesn't matter that they are sisters, or if they are married, parent/child, etc. I couldn't find anything about how many pc's the game is licenced to be installed on, but then again I couldn't find the End User Agreement (after a very brief search), nor did I have enough time to properly look through EA's ToS.
Sorry, sounds all doom and gloom. However it's not all that bad. Nobody is going to come and tell you off for lending your game to your sister, etc, and most companies expect a person to install their games on more than one pc (i.e. desktop and laptop), and probably EA do have a statement to this effect. It's like when music first become digital on CD, it was illegal to copy your CD onto different devices i.e. you were allowed to play the CD in different devices, but you couldn't hold a copy of the CD on, say, your pc, and your mp3 player, etc. But that was soon determined to be unfeasible and thus the licencing laws were changed.
Hope this clears some things - and sorry for the long post.