Only one of you can claim a dependent.
If no one person (or married couple) provides 50% of the support (for example your siblings are also sending support), then a "multiple support agreement” (IRS Form 2120) can be used, to allow you to claim the dependent. https://www.irs.gov/pub/irs-pdf/f2120.pdf
You and your sibling simply need to agree on who will claim her. You can also alternate every year.
You can't each be providing more than half of her expenses. At best you can each be providing exactly half her expenses. But even that is unlikely, if she has any money of her own.
What about the 2020 stimulus ($1200 + $600)? Did she already get that. If not, it may be best if nobody claims her on a 2020 tax return, so she can file for the $1800.
