Complete this mission before you head to The Isle of Mists. It becomes available after Ugly Baby.

There are three Brothers in Arms quests all in all. The Velen character quests involve the following two:

1. Keira Metz: Complete Keira's quests, An Invitation from Keira Metz, A Towerful of Mice, A Favor for a Friend, and For the Advancement of Learning to ensure Keira is hiding safely in Velen and will join you.
2. Letho: If you've completed The Fall of the House of Reardon and Ghosts of the Past and Letho is alive, you can recruit him in Velen.

Visit the two characters to get their pledge.

Next, complete Brothers in Arms: Skellige.