-Your sister is not counted into your family member (household size), but your spouse is.
然後他們兩個人各填一張864A,是嗎? 謝謝。
-If your spouse has enough income, you don't need your sister's sponsor. If you need both your spouse and your sister's support, then you need to file I-864 and your spouse files I-864A as your household member, and your sister files another I-864 as joint sponsor.
By the way, your spouse and your sister must be green card holders.