You're right about German wieder and Old English wiðer 'against', but the shins is cognate with OHG sint 'way, side', sinnan 'to go, to strive', sindon 'to travel' (from a PIE *sent- 'to go, set out; perceive'. The one citation I could find for withershins in OE is wiðersinnis, which is why -- I guess -- most dictionaries say the word is borrowed from MLG. I misread the entry in the OED to say the shins part was related to English sun, but it's not.

There is a Widerschein in German that means reverberation. There's a good online German-English-German dictionary here: http://dict.leo.org/.