That's all very well and to the benefit of your son, but is fairly irrelevant where your daughter is concerned.
It's not a failure on your part to reassure her and trust her to do the work necessary - the absence of pressure to go higher than a 5 can make it more likely that she will do far better than that, as she isn't put in a mindset that identifies anything less than 12 x 9 as abject failure.
She could also be saying this not necessarily out of a belief she isn't capable, but as a way of taking pressure off from you (and herself, internal perfectionism is the enemy of good mental health). It could therefore be an extremely intelligent thing she is doing in trying to manage your expectations in advance and recognising what happens to others when a single grade 8 is seen as a complete disaster.
If she picks courses that interest her and she enjoys, it is highly likely that after a term or two, she realises that actually, she's more than capable of higher grades and will automatically work harder - not longer, more effectively - because she has developed confidence in herself and her subjects, particularly if she doesn't have the spectre of her mother saying she's failing/getting less than her sister/isn't as good as her sister/isn't good enough.
Take a position that she's not good enough, that she has to be perfect or you think she is a failure compared to you/her sister/what her brother would have been had he not also needed an EHCP and she's likely to buckle under the pressure. You don't want her to be able to say 'My mother? No, I was never good enough for her, everything was always ''why can't you be more like your sister?'' '.
Keep a narrative that you have confidence in her, trust her, love her, want her to be happy and all will be well.