Before only Minor Buffs affected companions, things were changed in Update40 to exclude pets like the Clannfear and Bear from group buffs while including companions as valid targets for Major Buffs such as Frost Cloak and Molten Weapon, I assume sets that grant group buffs also affect them too. My guess this is intentional as players do sometimes use companions as filler replacement party members for group dungeons, trials, and arena; why shouldn't they also get those buffs to aid them.

Now when you cast Molten Weapons they not only get 10% increased damage from Minor Brutality, they get an additional 20% from Major Brutality as well and their damage reflects that change. The On-Fire is normal as it affects your weapons as a player, companions count as a whole not just their weapons so their entire sprite is the attachment point.