I would replace nobody from your team right now. You don't have that many debuffs at all that FB's poison sensitivity isn't placed if you - as allready recommended - don't use the bad toxic set anymore.
On the long run you should replace Kael, but not now, because he is one of the very few lvl 60 champs and you probably skilled Warmaster on him, so he does the most damage right now. Any champ with lvl 60 can do that. You have a fantastic CB champ unascended and unused in your roster, Jareg. You should replace Kael with him as he reaches lvl 60 in the future. He doesn't do much damage on his own, but he is keeping your team alive much longer. Of course Aothar and FB should reach lvl 60 as well.
Like I allready wrote before, the low damage of FB doesn't indicate that she is useless. Her poison sensitivity increases all poison damage. She increases Aothars damage with her skill, the screenshot shows the benefit she brings to your team as his damage. His poisons will do 25% less damage when you remove FB. And when you make her lvl 60 and skill Warmaster on her, FB's damage will increase a lot.
So your path should be: 1. FB lvl 60. 2. Aothar lvl 60. 3. Warmaster/Giantslayer on everybody in your team. 4. Jareg lvl 60, ascended, Warmaster skilled. 5. Jareg replacing Kael.