HI, in the case of the element A containing multiple sentences (assuming
"." as end of sentence punctuation), is there a reliable way to find the
sentence that surrounds the child element B wherever it occurs in A?

I think that the solution (regex?) will have to look backwards from the
start tag of B and past the end tag of A to the nearest "."

I recognize that if there is some abbreviation or decimal number in the
sentence that will be interpreted as the end of sentence. That's OK as a
limitation.