Complex sentence: Welby had a direct effect on the Significs group, most of whose members are Dutch, including Gerrit Mannoury and Frederik van Eeden, and thus she indirectly influenced L. E. J. Brouwer and the founding of intuitionistic logic.
Split into simpler sentences: Welby had a direct effect on the Significs group, most of whose members were Dutch, including Gerrit Mannoury and Frederik van Eeden. Hence she indirectly influenced L. E. J. Brouwer, the founder of intuitionistic logic.