Just selecting the sentence with the insertion point in it:
If Selection.Type = wdSelectionIP Then
Selection.Sentences(1).Select
End If
Caveat: Word's idea of a sentence includes the spaces between this sentence and the next. This may be more than one, or something you do not want at all. Also, if the sentence is the last sentence of a paragraph, the paragraph mark will be included in the selection plus any following "empty" paragraph marks.
IS