I don´t know what was Sinister´s point there I get his actitude of "keep her away from Havok" an oppinion Magneto himself would share given their story but the why exactly is unclear:
Because Lorna had a normal unbringing with the Danes? Because she´s Magneto´s daughter? or it was a jab at Magneto for not raising her himself? I get Magneto got insulted on Lorna´s behalf but for Sinister those reasons are quite weak and out of place, unless like with Scott he has a chosen romantic partner for Havok and doesn´t want Lorna getting in the way?