Sibling test may find your biological sister even without participation of parents.
Search for the biological shared father
If sisters share the same biological a full siblingship can be tested with the determination of a biological father.
While women possess two X chromosomes (XX), men have one of each, one X and one Y chromosome (XY). So men will always inherit their father's one X chromosome while the mother will pass on one of her 2 X-Chromosomes. Accordingly, in a sister full-sibling test you can determine a shared biological father by analyzing the X-chromosomes the sisters inherited. This test can be more conclusive if the mother participates.
Search for biological shared mother
If the question is whether the sisters share the same mother we can conduct a classical maternity test including the mother's samples. If the putative mother is not available for testing we can perform a sibling test using the mitochondrial DNA