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Auto populating sibling records field values based on other siblings

Need help on below requirement. Really appreciate your help.

I have two objects.
Parent: Phase (fields, Name, Start date and End date)
Child: Task (Start ,end date)
If and only If all tasks are completed, then i should fill my phase "end date " field..
Phase    Start      End
Phase-1 1/2/2015 2/2/2015
  Task End date
  Task1 1/5/2015
  Task2 1/11/2015
  Task3 1/25/2015
  Task4 2/2/2015

Phase-2 2/2/2015 3/2/2015

After completion of last task my phase end date should be auto populated with last task completion date.
Phase2 will start only after phase1 and I  should auto populate start date of Phase 2 from Phase 1 end date.

Thanks in advance for help and time.