Complex sentence: According to the UKB website, its members are mostly descendants of "Old Settlers", Cherokee who migrated to Arkansas and Oklahoma about 1817, before the forced relocation of Cherokee from the Southeast in the 1830s under the Indian Removal Act.
Split into simpler sentences: According to the UKB website, its members are mostly descendants of "Old Settlers", the Cherokee who migrated to present-day Arkansas and Oklahoma about 1817. This was before the forced relocation of Cherokee from the Southeast in the late 1830s under the Indian Removal Act.