Such a seclusion is basically allowed and there is not a ruling that forbade for male and female relatives to be alone in seclusion and to travel (alone together). This is on condition that it is assured that there will not be problems of temptation, but if there is a fear of temptation, they are to be treated like foreigners in the case of seclusion and traveling together alone.
If the age of both relatives are close, there is no need to be worried about temptation. However, they are closer to temptation from other older adult relatives especially when the relatives are of youth age. It is better to be precautious. When it is assured that there will be no temptation from both sides, there is nothing wrong in them being in seclusion and it is not forbidden for them to travel together when the needs arise.