If a quote is rife with errors, can a single [sic] be used at the end of the quote rather than after each error?
She wrote, "Your [sic] making me weigh [sic] to [sic] sleepy."
She wrote, "Your making me weigh to sleepy [sic]."
You should use [sic] after each mistake, in case you introduce a new mistake.
For example, let's say we use the text you've used:
Your making me weigh to sleepy.
If we misspell making,
Your [sic] mkaing me weigh [sic] to [sic] sleepy.
We show that the misspelling was introduced by the quoter.
On the other hand, if we do
Your mkaing me weigh to sleepy [sic].
It attributes the misspelling to the quotee.