I have to wonder what Ian and Heather's relationship was like with Erin.
They had never been invited for a meal before the fatal (and near-fatal) one - and they were both the ones to quickly question why Erin's lunch had been on a different kind of plate, after they fell ill.
I know this is a bit weird but maybe Erin thought, if i invite these two along its going to look less like i am trying to kill anyone.
In her mind she may think the reason she is wanting kill her mother and father in law looks too obvious, so she try's to make it look less obvious by inviting Ian and Heather (who perhaps she secretly doesn't like anyway).
Purely speculating and MOO.