It means 'if at any time' or 'if at all'. The meaning of the whole sentence is "We see them very seldom, maybe we even never see them."

"If ever" means "if we've ever seen them". The whole sentence means "I'm not sure whether we've ever seen them. Even if we have, we don't see them many times."

"We see them very seldom, if ever." In this context it means that the odds of seeing 'them' are very low, so low that it would be surprising if you see them at all. You might say instead, 'We nearly never see them' or "We hardly ever see them". I hope this helps. Emma - Talk2Me English