[Doitsu]

It's highly unlikely that someone is going to write a full citation manager plugin for Sigil.

Assuming that CSL files that the OP works with look like my simple json sample, it would theoretically be possible to create a simple plugin that converts the CSL .json content to an .xhtml file with citeproc-py and adds it as an endnotes section to the current epub.

JSON:

Spoiler:

[
    {
        "id": "cite1",
        "type": "book",
        "author": [
            {
                "family": "Smith",
                "given": "John"
            }
        ],
        "issued": {
            "date-parts": [[2020]]
        },
        "title": "Understanding Complex Systems",
        "publisher": "Academic Press"
    },
    {
        "id": "cite2",
        "type": "article-journal",
        "author": [
            {
                "family": "Jones",
                "given": "Alice"
            },
            {
                "family": "Brown",
                "given": "Thomas"
            }
        ],
        "issued": {
            "date-parts": [[2018]]
        },
        "title": "Advances in Research Methods",
        "container-title": "Journal of Science",
        "volume": "45",
        "issue": "3",
        "page": "123-145"
    }
]

HTML:

Spoiler:

<section epub:type="backmatter" id="endnotes">
	<h2>Endnotes</h2>
	<ol>
		<li id="endnote1">
			<p>Smith, J. (2020). <em>Understanding Complex Systems</em>. Academic Press. <a href="#cite1">↩</a></p>
		</li>
		<li id="endnote2">
			<p>Jones, A., & Brown, T. (2018). <em>Advances in Research Methods</em>. Journal of Science, 45(3), 123–145. <a href="#cite2">↩</a></p>
		</li>
	</ol>
</section>