The Lt Governor's office had earlier recommended a CBI probe in the matter after she was appointed an advisor in the health department.
The Delhi government, on the direction of the LG's office, had sent a letter to CBI in December last year to look into the matter.
Jain had earlier rejected the allegation of favouritism, saying she had worked voluntarily and not a single paisa was given to her.
In July, Soumya had resigned from the post following the opposition's allegation of nepotism.
Official sources said the Preliminary Enquiry will cover the role of Jain, the MD of the Health department and others, and will decide if a regular case needs to be registered.
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