Then why not take a younger female sibling ?
For the reasons I have outlined on several occasions.
Assuming an abduction happened, which I cannot come close to proving, then we are left with 2 motivations.
Paedophiles or replacement for a lost child.
In a report around about 2000, the Portuguese intelligence service summarised that there were 4 centres of paedophilia in Portugal, and 3 international paedophile rings in action. The 4 centres were Lisbon, Oporto, Madeira and the Azores. The 3 rings were not identified, but the word that got my attention was 'international'.
Replacement? Look at the MW arrivals sheet and it is clear. Madeleine was 3. She was not days off being 4, she was 3 and appears to have been petite for her age. It is true that someone subjecting her passport to scrutiny on her arrival could have twigged she was pushing 4, but it would have required a deal of scrutiny.
Alice has mentioned Schengen, which may be pertinent.
Take a child over the border to Spain, no passport required, no check, and the PJ are stymied. Take a child to Madeira or the Azores, no passport, no check, then away by boat, and equally the PJ are beaten. If it was pre-planned, it was easy to leave the PJ trailing.