I couldn't bring myself to do it knowing that it causes Tova's suicide and they already were mourning for their other daughter.

But I also couldn't figure out a good reason why my character wouldn't accept Muiri's offer.

Do you think your targets don't have a family or friends that will suffer for their loss? It would be a great contradiction if that were the reason why you didn't agree to kill her. Maybe your character thought Muiri's reason for wanting her dead is too petty, but why pass up on the coin anyhow?

I still didn't kill Nilsine, but that left a little hole in my RP.

Yes. I could care less for the Shatter-Shields to be honest.

As a paid assassin... the outcome of your actions should not be your concern. You should have made the decision based on what your character feels. And your character would have NO idea that the mother would kill herself.

If I'm an assassin, I've no qualms about killing ANYONE !

It's a job, nothing personal.

I'm in it for the dough, eh. besides, Muiri looks a 1000 times better than Nilsine, so...

Maybe, Maybe not. Depends.

I did it on my first DB playthrough, but when I learnt her mother kills herself and that she already has a dead sister my heart broke. I felt awful.

Freebies? Huh? For the first kill, you're getting a levled amount of gold and for killing Nilsine you get Muiri's ring (a silver ring with a fortify alchemy enchantment).

Yes, it's part of the job and to not do so will piss off the dread father Sithis.