There are a few odd things about Ellie and Daniel's timeline In "The Variable" that i found really interesting. we see Ellie pregnant in 1977 (at 40 years old), then mother to 10 year old Daniel (presumably at 50), then switched to a new, much older actress at his graduation from Oxford around 1994. If it all happens in a straight line then Daniel is only around 16 at this graduation, Ellie (at 56) has aged more in the 6 years since the piano scene than she did in the 10 years prior, and Theresa would be a grown woman dating a teenager.
The only way around this is if Daniel was born prior to 1977. A birth in 1974 or earlier would make him at least 20 at the graduation. A similar line of reasoning can be applied to Penny, who met Desmond in 1994. She too would likely be 20 or older at that point, and would be born in 1974 or earlier. But then the question arises: where and with whom were these children being raised when we see Ellie and Charles leading the Others? If Charles had really never met Daniel Before, Ellie must have left the island before giving birth, soon after the incident. If she shifted 3 or more years into the past during the process, it neatly resolves the pregnancy with the age issues. Daniel can be at least 20 in 1994, and Ellie can be at least 60.
I think It's notable that the actress cast as middle aged Ellie looks quite a bit like Penny, with similar hair, high cheekbones, strong jaw, and blue eyes. When Ellie first talks to Penny at the hospital she seems a bit emotional, which is uncharacteristic considering her all-business manner with Desmond in the past. The emotion seems to be directed toward Penny and little Charlie, at whom she sighs. When Penny asks if she knows her, Ellie seems to gather herself. If Ellie is Penny's mother too, then the similar ages of the characters might mean they're twins, separated at an early age. Obviously there's still the possibility that Charles had an off-island relationship with an unknown character, but so far we've only seen one relationship for him, and one pregnancy for Ellie, so it might be two buns in that oven.
Another funny thing from 'Dead is Dead"- when Charles was exiled, his crimes were "having a Daughter with an outsider" and "making frequent trips off the island", indicating that the Others were either unaware or unconcerned with him having a Son with an outsider. This may even mean that Charles was unaware that he had a son, or that he was hiding Daniel from the Others.