Clarification of conditions for 100% completion of missions (Assassin's Creed: Syndicate)

Greetings, comrades!
It seems that everything went exactly 100%, but there was a misunderstanding with the mission to eliminate the Templars "Children of the Slaughterhouse".
There, under the conditions that a sister must kill her brother for 100%, but how to do this is not clear. Who has already passed - share your wisdom =)
PS Also in the subject, leave your misunderstandings - if you have already passed, I will definitely help.