I just made my will. If someone is listed as a beneficiary, can that person sign as a witness? I've left my sister 20% of my assets and I also want her to be one of my witnesses.

Congratulations on taking charge of your estate planning and making a will, but your sister should not sign it as a witness.

Anyone who will inherit property under your will should not be a witness to it. This requirement prevents witnesses from coercing will makers into leaving them some property. If later called to court to testify about the state of mind of the will maker, witnesses receiving property would obviously have reason to fabricate a thing or two. Better to choose any other person over 18 and of sound mind as a witness. Don't worry, the person doesn't have to read your will, just observe that you're the one signing it. For other information on what not to do with a will, see What a Will Won't Do.