My father receives Social Security benefits because of an injury he sustained 10!

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They are her benefits (child's benefit), not your father's. His will continue as before, hers will stop when she is 18 or 19 and is no longer attending high school:

"Benefits can continue at age 18
Benefits stop when your child reaches age 18 unless your child is a student or disabled.

If your child is a student
Three months before your child's 18th birthday, we will send you a notice that benefits will end at age 18 unless your child is a full-time student at a secondary ( or elementary) school. If your child is younger than 19 and still attending a secondary or elementary school, he or she must notify us by completing a statement of attendance that has been certified by a school official. The benefits then will usually continue until he or she graduates, or until two months after reaching age 19, whichever comes first."

http://www.ssa.gov/pubs/10085.html#who

That extra benefit for your sister's care is based on your Dad being unable to work and take care of his minor children. Yes, when she turns 18 and is no longer a dependent minor, the extra benefit ends. That money is only for your Dad to spend taking care of a minor child until she is 18