It is true that the Christians later adopted the Celtic cross as it fit with the crucifixion. It is just an example of the integration of pagan symbols and ideologies. The symbol was originally indicative of the compass directions and the four elements surrounded by the circle, a symbol of infinity, cycles, and reincarnation.
I like the Ankh idea also. The two phases of the moon (new and full) would better represent the two clans as the Dark Ones rule over the waning time and the Wise Ones rule over the waxing time. A branch would be fine, except I picture a fruit bearing branch as the Wise Ones are vegetarian and tend to orchards and gardens. Maybe a sheaf of wheat?