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The relative lack of political centrality that museums have is a direct consequence of the depoliticization of the sector that has arisen from the importance attached to the range of technical and professional assumptions and practices that are seen to be central to their ability to exercise their functional roles. This depoliticization allows museums and their staff to exercise their ideological and legitimating roles in relative isolation.

this means that museums are depoliticized, so they are able to exercise their own legitimating roles without other’s interference?

What is in relative isolation?

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Here, "in isolation" means without others' interference, as you say.

"relative isolation" means isolation when compared to people in some other (unstated) roles, as opposed to complete (absolute) isolation. In practice, though, when the object of the comparison is not specifically mentioned, "relative" tends to have the effect of moderating the following word, rather than strongly focusing on a comparison.