The 1852 LDS edition accidentally replaced the pronoun it with this, probably under the influence of the preceding this in “from whence cometh this blood”. The 1920 LDS edition restored the correct pronoun it to the LDS text. Either reading is, of course, possible. In the text, there are 33 examples of “that it is” (including this one in Helaman 9:32) and 21 of “that this is”, where that is the subordinate conjunction.
Summary: Maintain in Helaman 9:32 the original reading with the pronoun it rather than this in “do we not know that it is the blood of your brother”; either reading is possible, but the earliest textual sources support the it.