FWIW, to capitalize the first word in a sentence (using Sigil) is quite straightforward. For the example below, I would have defined CSS classes "indentoff" (whatever) for the first sentence following a scene break, and "caps" to transform a string to capital letters. So:
Find:
<p class="indentoff">(.*?)[space]
will find the first text string after "indentoff" followed by a space. Note: [space] here represents a typed space; it is not part of the Regex!
Replace:
<p class="indentoff"><span class="caps">\1</span>[space]
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