Hello and welcome, good to hear you're enjoying it!
Racing your brother as a beginner doesn't seem like the best idea. Sorry to hear you got hurt, hope it's nothing serious and heals fast. Take good care of your knees, you're going to need them!
Riding (and turning) with your knees bent is very good. The people riding straight-legged are either doing it wrong or have their own reasons for doing so
I'm in my second season and I'm STILL learning. So yeah, the curve is a bit steep when you start out.
What helps with soreness? Hmm... stretch... drink lots of water during exercise (had someone tell me it would deal with some of the lactic acid that makes your muscles sore afterwards, don't know if true or full of shit...), warm up before snowboarding, and... I don't really know any more. You get a lot more sore from falling, your neck gets especially sore, so that should get better as you get better at it.
And finally, may I suggest taking a lesson or two to get you properly started? It helps A WHOLE LOT.