This sometimes happens between cousins who are not living close to each other.
They treat each other as normal friends. These kind of relationships are frowned upon in many societies and there is scientific evidence to support the claim.
It depends on partners, their financial status, mental status and emotional maturity. Further how deep is love. Many factors are there. In teen age such flings can be disregarded as hormonal twists of nature.
Teenage love is strange. It is very reliable as well as unreliable!
The best and the worst love occurs in this age only. Some people fall in deep love while others just change with time. Hormonal imbalance you see.
A lot many factors to be considered.