I suggest to get a text string using the functions from RVLinear.pas (they are better because they just insert a character in place of non-text items, except for tabs and tables) and use it to calculate this statistics.
If a table is allowed inside a sentence... What if this table cells contains several sentences?
Actually, I suggest the following way of calculation. Get text using RVGetTextRange(rv, 0, RVGetTextLength(rv)) (from RVLinear unit) and calculate sentences in this text.