Your sister has eloped with her boy friend on her own, she is an adult hence no case of kidnap or nay other offence can be leveled against her boy friend.
If the boy friend is not willing to marry her, then let she realise the fact and return to home, after which your parents would be happily change her mind.
However if she do not want tfo come back to your parents then you cannot force her because she is an adult and has right to choose her own life as per her will and wish.
She cannot be held responsible for your parents health condition, they have to realise the facts and carry on their life ignoring the sentimental issues attached to it.
These days or even in the previous days the inter caste marriages were common and any effort put by parents to stop them would repulse and would turn into an ugly situation sometimes may result into suicidal deaths too if the parents are very strict and do not recognise it.
Therefore if at all your sister is not traceable neither the boyfriend is not disclosing her whereabouts you can file a haebus corpus petition after lodging a man missing complaint with the concerned police,.