Complex sentence: Due to them being born in a different reality to their conception, both of them grew to be eighteen years old within three days, so to make sure they did not die within a fortnight, Lister sent them back to their mother's dimension.
Split into simpler sentences: Due to them being born in a different reality to their conception, both of them grew to be eighteen years old within three days. Realising that if they stayed with him they would be dead within a fortnight, Lister sent them back to their mother's dimension.