I'm trying to look into aspects of this family, and it looks as though Elizabeth Ergham/Erghom/Erghum should be a full sister of Sibil, rather than half.
I'm still looking for a good solid academic source to confirm this, but the suggestion is certainly that the two sisters shared Sibyl FitzHenry as a mother.
I don't have pre-1500 rights so can't change anything anyway, but I'd welcome the views of others.
This may shed some light, but I can't access it here in the UK.
There may also be something in The Complete Peerage, by Cokayne, Vol. V.
On 14 May 2018 Anonymous Horace wrote on Erghum-2:
Could be sister of Sibyl [[Erghum-1]]