Complex sentence: He started to attend Petrashevsky's' Secret Fridays' and maintained close contacts with Fyodor Dostoyevsky and Aleksey Pleshcheyev, becoming effectively a member of the Petrashevsky Group which to even greater extent had his brother Valerian involved.
Split into simpler sentences: He started to attend Petrashevsky's' Secret Fridays'. Maintaining close contacts with Fyodor Dostoyevsky and Aleksey Pleshcheyev, he became effectively a member of the Petrashevsky Group, which had to even greater extent his brother Valerian involved.