I received Mixcraft as a gift over the holidays. I've used Audacity. I imagine every software has a learning curve. You can do MOST of the same things in Audacity that you can do in Mixcraft, it's just not "as pretty" and in some ways not as easy, but in others even easier. Frankly, I haven't played with it enough yet to feel comfy with everything it can do, but it sounds like it's a whole lot more than you need!
If he's doing all the mixing, you certainly don't need Mixcraft on your end or any program for that matter. All you need is a way to record your stuff and send it to him. Unless you plan on doing a lot of mixing of your own stuff down the road, spend your money on a nice recording device instead because they come in handy for lots of uses! I have a Zoom recorder. It records wav files, can normalize files, and convert to mp3. If you don't need to do any mixing yourself, then that's all you need!
On another note... if you don't have something like a Zoom recorder, how do you plan to record yourself? Using a standard Computer Mic with Mixcraft is likely not going to do the trick. As an example, I've tried using my Zoom as a USB mic and go directly though Mixcraft to record on the computer, and I have issues (my computer sucks). So, I generally record on my Zoom and then transfer the files to the computer to play around in the software. Really, that's all your brother wants. Send him recordings so he can mix. You don't need any program for that, just a decent device to record with. 
You can certainly try Audacity and whatever devices you have laying around that you can record on plugged into the computer. You'll get the same quality as you would have if you'd done it through Mixcraft for the price of free. I still swear by my Zoom though. It's what I used to record the Virginia Chordie gathering out in the woods running on batteries.
Art and beauty are in the eyes of the beholder.
What constitutes excellent music is in the ears of the listener.
but would the 2 systems be compatible? Like I said, he has already gone through the learning curve of Mixcraft. He's been using it for a couple years so I want to be sure whatever I use will work when I send him files. For all I know, all the different systems may work together but I don't know that to be a fact.
