Illegal possession of a firearm in Pennsylvania can be graded as a felony or a misdemeanor and is a very serious offense. If the weapon was loaded the sentence could be very harsh. Therefore, whoever is charged here, they need to retain an experienced criminal defense attorney. In all criminal cases the Commonwealth must have evidence and must prove the case beyond a reasonable doubt. The attorney will do everything possible to put the Commonwealth to its burden.
Theft of a firearm is graded as a Felony 2, which is punishable by up to ten (10) years in prison. Many factors go into determining a sentence only one of which is revealed in your question. The offense described is serious and has the potential of a significant period of incarceration. What ever person(s) are charged in this matter will need the assistance of qualified criminal defense counsel. Both siblings need to retaining counsel now, even if neither has been charged yet. As has been said already, the Commonwealth needs evidence establishing guilt beyond a reasonable doubt. The testimony of a single witness such as the sister, could be sufficient evidence to support the charges against the brother.
If i understand this, some guy's sister stole a gun, sold it and told the police the guy did it. If that is the question, yes, the police will need evidence to prosecute him. Is there any other evidence besides the sister's testimony? Does the sister have prior crimes that would be admissible in a court of law against her? There is too much i don't know here, but if the case against him is his sister's word only, he has a good chance of beating it.