My Mum's family is from Belfast. She moved to Australia when she was 30, had children, we obviously speak with an Aussie Accent. Her older sister also migrated here a while after Mum did, along with her husband and 2 children who were 9 & 11 at the time. Both of the kids still have a distinguishable Irish accent, the oldest is slightly stronger.
I don't notice it as much as I'm used to the accent, but other Aussies do very easily.
As their main influence at home was Irish, and they also had quite a few family friends who spoke the same.
From the show it appears Siobhan also has a few UK ppl in her life, so it's reasonable to think they were around them. It also seems pretty reasonable to think that since they were in hiding, they didn't do a huge amount of socialising outside of the home, especially early on. We know that Sarah didn't complete her High School education, and was a bit of a trouble maker, so it's also possible she was only in school a couple of years and didn't have that daily interaction in the classroom to influence her accent.
I actually don't even find her accent that strong for a South London accent (which based on what we know is what it's supposed to be). Sounds a little stronger when they're using slang etc, but from spending a few years int he UK myself it's certainly nowhere near as strong as some. So it possibly has softened quite a bit.
Sure, people who move at that age could possibly lose their accent, but it is also extremely plausible they would not, especially for a strong independent punk rock character such as Sarah.
So since it obviously lies withing the realm of possibility, I don't see why it's an issue...
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