Winthrop Federal Credit Union is now part of Webster First Federal Credit Union. The merger was approved Oct. 26 by the National Credit Union Administration, and finalized Dec. 14 after a vote by Winthrop Federal members. It took effect yesterday. The merger follows two other recent acquisitions by Worcester-based Webster First. In May, it acquired Saugus Federal Credit Union, and in November, Fitchburg Federal Credit Union. With the addition of Winthrop Federal, Webster First now has more than $600 million in assets, according to Katelyn Andrews, the group’s marketing coordinator. She said for Winthrop Federal members, the merger offers the opportunity to make use of Webster’s 16 branches.