You can file a complaint on the boy and family that they forcefully took your sister and abducted her. Further boys side cannot file any complaint on you as such or family.

See in case sisters statement are recorded or proof is furnished of there love affair the police won't file FIR but shall take complaint and give receipt of same.

When you know, your sister is a major and has eloped, where's the question of abduction?

You should register Missing FIR in your jurisdiction Police station with all facts. 

1. I suggest you to file a missing complaint with nearby women police station. You may also disclose the information about suspected persons.

2. Based on the prima facie evidence, police would register FIR. In case, If police refuse to register FIR then you may approach a lawyer to take up your complaint to court.

3. You may also send the letter to external home affairs ministry and the embassy along with the FIR details. 

4. As of now, you would not need to take any steps to safeguard yourself.

Dear Client,

You can file missing complain and habeas corpus petition in High Court but as it will revel to police & court that she willfully eloped, both will drop proceeding.

She is major and cannot be forced against her will.

No case agasint your family, you can file criminal complain against boy that he kidnapped your sister.

1. If your sister has voluntarily eloped with a guy, then no action can be taken, since she is major in age and is independent to take decision regarding her marriage.

2. If your sister was forcibly abducted by the guy, then you could have lodged a abduction  complaint against the guy.

3. To safeguard your family from any case which may be filed by the boy's side,  lodge a missing complaint in the Police Station well before the boy's side initiate action.

1) your sister is an adult and liberty to marry any guy she wishes 

2) no missing case or abduction complaint is maintainable 

If your sister is major and run away from the family on her on unmarried a living with  him voluntarily then any case filed by you will be dismissed. But in case you feel that she is abducted and kept Hostage then you can file habeas corpus writ petition in the High Court where is your sister will appear and make a statement in front of the court where she wants to go.

1. It will not be of any use to  lodge a complaint in India alleging abduction of your adult sister abroad which is beyond the jurisdiction of Indian Police or Court.

2. You shall have to lodge the comp-laint before the police of the said foreign country who might take your sister in to custody for interrogation and if it is found that she has left with that person of her own, she will be released and you shall be warned against disturbing them.

You clearly know that she eloped with a guy that too to abroad, then do you think that will there be any effect on the complaint you propose to make now on this subject?

You can lodge a man missing complaint with the police, but that will remain idle if the police comes to know the fact by enquiring the neighbors.

If she has eloped on her own wish then you can fo nothing. Because she is adult. Even if you file kidnapping complaint then if after coming back she confesses that she eloped on her own then it will backfire on you.

Dear Sir,

Immediately you must file such complaint and keep acknowledgment with you to avoid future complications.