despite Europeans being lower than Arabs and Turks in racial hierachy?
Several Ottoman sultans had Caucasian wives, and several Sultans were part Caucasian.
From the memoirs of Princess Emily Ruete:
One Circassian woman, favoured with an aristocratic appearance, was avoived and even hated by the chocolate-coloured Abyssinian women through no fault of hers, but simply because she looked majestic. Under these circumstances it was bound to happen that occasionally a kind of ridiculous "racism" broke out among my brothers and sisters....We, the children of Circassian women, were usually called "cats" by those of our brothers and sisters who had Abyssinian blood in their veins, because some of us had the misfortune of possessing: blue eyes.Derisively they called us "Highness", a proof of how annoyed they were about us having been born with lighter skin. My father was of course never forgiven the fact that he had chosen his favourite children Sharîfe and Chole—both by Circassian mothers, Sharîfe even being blue-eyed—from the hateful race of "cats".
According to the feminist Harriet Martineau, Circassians were the only saving virtue of the Egyptian harem where these Circassian mothers produced the finest children and if they were to be excluded from the harem, the upper class in Egypt would be doomed.
sources:
DelPlato, Joan (2002). "Chapter 2. The Colonial Contexts for the Harem Representation"
An Arabian Princess Between Two Worlds: Memoirs, Letters Home, Sequels to the Memoirs
Several Ottoman sultans either married or kept as concubines European women. However since your question is based on this idea of a "racial hierarchy," I'm going to have to ask you to explain the background of your question a bit more - specifically where did you pick up this idea and how was it explained to you?
The Ottomans had no distinct conception of a racial hierarchy, indeed the Ottoman elite was highly ethnically mixed, and they were conscious of that fact. Rather than imagining themselves as part of a singular "Turkish" national-ethnic group, the more common self-identity of the educated Turkish-speaking population of the Balkans and Anatolia was "Rûmî", meaning literally "Roman" but referring in fact to the territory of the former Byzantine Empire. And Rûmîs were made up of people of many origins, both Middle Eastern and European, as the 16th-century historian Mustafa Âli put it:
Those varied peoples and different types of Rumis living in the glorious days of the Ottoman dynasty [...] are a select community and pure, pleasing people who, just as they are distinguished in the origins of their state, are singled out for their piety, cleanliness, and faith. Apart from this, most of the inhabitants of Rum are of confused ethnic origins. Among its notables there are few whose lineage does not go back to a convert to Islam... either on their father or their mother's side, the genealogy is traced to a filthy infidel. It is as if two different species of fruitbearing tree mingled and mated, with leaves and fruits; and the fruit of this union was large and filled with liquid, like a princely pearl. The best qualities of the progenitors were then manifested and gave distinction, either in physical beauty, or in spiritual wisdom.^1
So in Mustafa Âli's view, it was from the mixing of various peoples within the Ottoman Empire that emerged the population of Rum's uniquely virtuous qualities. That's not to say that racism didn't exist, indeed members of the Ottoman elite did at times form relationships and cliques with one another on the basis of their ethnic origins, and negative stereotypes existed about, for instance, Albanians and Abkhazians.^2 But there was no "hierarchy" as such, and no conception of any kind of need for racial purity.