It is illegal to record a conversation in Pennsylvania without the permission of all parties involved. See 18 Pa. Cons. Stat. §5703. These laws are so strong, that even a police officer was found guilty of the offense when he recorded a routine traffic stop in PA. Consequently, a security camera that captures sound could be illegal if it picks up a conversation you are having by the fence with someone else for instance. It's actually a 3rd degree felony and allows one to collect civil damages in excess of $1500 or actual damages, which ever is greater, although I am not certain of the precise dollar amount.
Pennsylvania also recognizes the civil tort of invasion of privacy. The question is whether an average person in the community would find the act to be unreasonably intrusive. This will depend on what your neighbor is able to record from the angle of the camera. If it is merely a portion of your backyard, this may not be an unreasonable intrusion. If the camera records conduct within your home, this could be an unreasonable intrusion.
The Pennsylvania Constitution guarantees a greater right to privacy than the rest of the country. This is because our constitution is older than the federal document. Your best course of action would be to inform your neighbor that Pennsylvania privacy laws prohibit audio recordings without the permission of the person being recorded and that video recording that constitutes an unreasonable intrusion is a tort that will make the responsible party liable for damages. Your neighbor can legally record her own property without notice, but recording other people with or without notice can lead to legal trouble.