I'm thankful for your concern for your sister. If she is involved in a cult, you need to know this and urge her to break her ties with it, because cults can be very harmful to a person's emotional and spiritual health.
However, just because her group is small and not as well known doesn't mean it is necessarily a cult. God is at work in many churches and denominations today—including some He has raised up only recently. Whenever the Bible is truly preached, lives will be changed and Christ will be honored. Jesus told His disciples, "Whoever is not against us is for us" (Mark 9:40).
In general, cults differ from Christian churches first of all in what they believe. Cults always deny some part of what the Bible teaches (which is why some add other so-called "sacred" books to the Bible). Many deny the deity of Jesus, or believe His work on the cross wasn't sufficient to save us from our sins.
Cults also differ from Christian churches by the way they act. Cults often stress absolute and total obedience to their leader (instead of to Jesus), and threaten anyone who wants to leave the group by claiming they will be lost if they do.
Find out more about your sister's group—and if Christ has truly changed her life, be thankful for it. Christ can change anyone who puts their faith and trust in Him—including you.