D.B.
I'd keep this really simple, and call your SIL. Don't make a lot of excuses or explanations - just say, "Hey, Susie told me she gave you the painting on the piano." (Wait for a response to confirm it.) Then say, "I know it might seem silly to you, but I had plans for that and am picking out a frame. I'd like it back. If you feel strongly that you want a copy, I'll be happy to stop at the office supply store and make a color copy for you. When's a good time for me to pick it up?" I wouldn't go on and on about how you feel so petty or that you're so upset.
It's sweet that your daughter wanted to give it to her aunt, and sweet that your SIL admired it and complimented your daughter. Stop there. Unless this SIL has a perpetual problem with interfering and snatching things, let it go. There's no reason to tell your daughter not to give away something she made - her toys and clothes, yes. But a picture she made? How in the world would anyone anticipate that one particular thing left lying about would be so special? It's fine that you really feel attached to it, but not okay that you expected everyone to know about it. Kids make dozens of paintings and we all feel badly getting rid of them - so it's reasonable to think your SIL would have no idea that this, of all paintings, was incredibly special and deserving of framing.
Take a breath, make a call, and keep the emotion to a minimum, lest you cause a bigger problem.