Whether Employee purchasing shares of fellow subsidiary company is RPT?

Employee doesn't come under related party definition as per Act. Unless he is employee in capacity of KMP. Further purchase of shares also doesn't come under RPT as given in Sec. 188 of the Act.

Employees are creditors and not related party

As per AS-18 a relative broadly includes-

1. Associate Companies
2. Subsidiaries
3. Fellow Subsidiary
4. Intermediary Companies
5. Controlled Companies
6. Holding Company
7. Key Managerial Personnel and there relatives
8. Individuals owning direct or indirect interest in Enterprise the voting power of the reporting enterprise or having significant influence over the reporting company or relatives of such individuals.