Is there any chance to do marriage for a 16 years girl and 25 years boy , if their parents health conditions are very serious and marriage is interest for both girl and boy and they are blood relatives ?
If they do marriage will it come under crime?
Not advisable and lawful under the Hindu Marriage Act 1955.
Marriage on compassionate grounds as stated in the question can be performed and it will not be null and void as such however the girl has right to get it annulled when she reach the age of 18 years.
POSCO Act comes to play if there a s*xual assault or act done with minor. In case of marriage of minor by itself doesn't mean s*xual assault or act being done with her.
Conjugal relationships in such case can remain suspended till the girl achieved the age of majority.