Join Date: Aug 2011
Location: Barcelona, London, on a plane
Programs: IB+ Gold, TK E+, Hyatt Globalist, Marriott LTP, IHG Diamond
Posts: 14,182
I'm not sure the exact technical term, but Air Nostrum is an affiliate of IB, in the same way that you might see a Delta Connection or a United Express operate regional flights in the US. So Air Nostrum = Iberia. (it's mostly about keeping employees off the mainline payroll / union)
If both flights are on the same ticket, you are protected. If not, then you will be at the mercy of IB.