The only threat to her comes, paradoxically, from within her own ranks. Since the second world war, the centre-right camp in German politics has had a peculiar arrangement: the Christian Democrats appear on ballots all over Germany except in Bavaria, where a “sister party”, the Christian Social Union (CSU), takes the CDU’s place. The two are independent but by convention campaign together in federal elections and form one group in the Bundestag.
I found 2 main definitions:
[ C ] a large formal meeting of people who do a particular job or have a similar interest, or a large meeting for a political party:
[ C or U ] a usual or accepted way of behaving, especially in social situations, often following an old way of thinking or a custom in one particular society:
Which one does it fit better in this case?