Sirius/Callum
What would I have to gain in doing that exactly? Especially when I've not taken any issue with my brother. If anything, I should be grateful that he kept our parents occupied for most of our lives to take the focus off of me and what I was doing.
If I were going to plan such a thing, I'd not get some half-wit excuse of an assassin wannabe to do the job, I'd do it myself and my target certainly wouldn't be Regulus.