An ambulence takes a wounded lady and her son to the local hospital. The kid waits outside the operating room while the doctors and nurses take the lady in. While the lady is being operated, a nurse comes and hugs the kid. She tells her fellow nurse that its her son (not step son, or nephew etc etc).
How is this possible?

फुच्चेको नाम हर्षन थ्यो होला नि त कीड्डो ब्रो !

 The wounded lady told the nurse that the kid waiting outside is her son. 

Kun chai Harshan ho thaha bhayena.

Riddle, milena. Read the question again.

 or a relative 

बिरामी हुने लेडी त्यो नर्स को समलिंगी पार्टनर हो , त्यो बाहिर पर्खिएर बस्ने नर्सको छोरो हो भने समलिंगी भएर बस्नु अघि बिपरितलिंग्गी संग सहबास भै जन्माएको छोरो हो ?

यसैमा एउटा थपी दिउ : बुहारीको भाईलाई के नाताले बोलाईने चलन छ ?

She tells her fellow nurse that its her (the wounded lady's) son.

MaGorkhe1 is right.

This is a twist in the original riddle that challenges the social norm that didn't see too many male nurse. Back then, society was just getting used to male nurses/female pilots. Now we are getting used to same sex parents and thus it throws us off sometimes.

For the case of male nurse someone who is good in grammar should be able to catch this easily.
In the case of male nurse the statement should be "He tells his fellow nurse that its his son".