sister's brother-in-law (sister's husband's brother)
brother-in-law's brother (sister's husband's brother)
I didn't repeat myself - there are two ways in English of describing your sister's husband's brother, and both should be in the dictionary, whether or not it's the same person!
>> How about "spouse's or sibling's sister-in-law"
>If it were this simple then my sister, who is my wife's "Schwaegerin" were also my "Schwippschwaegerin"...
Just the same is true of "wife's sister-in-law", but people wouldn't think you were talking about your own sister in that roundabout way :-)
"Asked by advocate depute Alastair Campbell QC where he was when his wife's sister-in-law had a baby in the early hours of December 22, Talb said: "The women went to visit the hospital and I stayed at home to care for the young children." " http://www.guardian.co.uk/Lockerbie/Story/0,2...
We *do* say "wife's sister-in-law" etc, it's just "spouse's sister-in-law" or "sibling's sister-in-law" that we don't say, BTW!