Originally it contained only 7 states and someone came up with this cheesy, feel-good term called “Seven Sisters”. Sikkim is the 8th state that was clubbed later, making it 8th state. This is because originally Sikkim was a small but independent Himalayan Kingdom, which joined the Indian Union only in 1971. It is the only state that is not physically touching the rest of the states. Still, it is not that far and some demographic and cultural similarities make it an ideal member.
Have a look at the map above. Sikkim on the left side si detached from the rest and the only way to reach Sikkim is through Bhutan or West Bengal. Nevertheless, it is now the 8th member of this group.