Please tell me, could a sigmoid volvulous resolve on its own?

Yes it can: If it is not completely twisted beyond point of no return. Otherwise, it needs to be reduced by endoscopy, and a surgical procedure is needed to prevent recurrence. But you should consult a GI specialist for specific recommendation, before this condition becomes a surgical emergency.

Yes, short term: This is usually related to constipation and is commonly seen in the elderly and severly disabled older children and adults. The colon gets very dilated because of constipation and can twist on itself. It can resolve but will recur if proper treatment of the cause does not happen.