You may petition for your sister right now, or same time that you petition for your mother. But your sister cannot be included as a derivative in your mother's petition. You will need to file separate I-130 petitions for them.
Even if you petition for both of them, only your mother can stay in the US, by filing an I-485 application. Your sister cannot file an I-485 and cannot stay legally in the US beyond the expiration of her current stay. She will need to wait for a visa number to be available to her before she can apply for an F4 (sibling of USC) visa from outside the US, maybe in around 16 years or so.
After your mother's I-485 is approved (I-130 approval alone is not enough), she may also file an I-130 petition for your sister. It's possible but unlikely that your sister will not age out of F2A (minor child of LPR) category, so I'm assuming that she will be under F2B (unmarried adult child of LPR) category if your mother petitions for her. So your sister cannot file an I-485 and cannot stay legally in the US based on the I-130 petition from your mother. She will still need to wait for a visa number before she can apply for an F2B visa from outside the US, maybe in around 8 years or so, after your mother files the I-130.
I assume your sister is here on ESTA? She must leave the US before her current I-94 expires. Because she is already 18 years old, she will accrue unlawful presence if she overstays in the US. Any overstay will jeopardize her chances of visiting the US in the future and could result in a ban.