I haven't gone as far to set up jamba to do attacks off the Marking ability yet.

But I see that you have targetlasttarget . Which means your slave is looking for your last target not your current one.? maybe?!

I would set up
/assist [target=focus, exists]
/cast Sinister

Now granted I'm a newb and still learning myself. So take what I have to add as a grain of salt. For a good example on my newbness --/targetexact #MOB#<--- this makes no sense to me =)

But the above macro works great for my 4 shammys.