The 3GS uses a normal sized SIM, not the smaller ones in some current phones. PIC.
Straight Talk is an AT&T MVNO, and others networks, but their relationship with AT&T might be going away.

If the phone was not an AT&T iPhone (I don't recall which carriers had the 3GS, but I think AT&T were the only SIM capable carrier with the iPhone then), that it will not work with an AT&T SIM, as it will still be carrier locked. Your BiL can get it unlocked as it is out of contract (it takes a week), which he should probably still do so your wife has carrier portability, either now or in the future.
She should be able to pick up a pre-paid SIM at any AT&T Wireless store, and put $25 credit on it, which will expire in 30 days. Note: This will give her 10c/minute calls, and 20c/text, but no data - so disable data on the phone.