a man killed his 2 sisters (one his own sister, another cousin) on account of unsocial deeds, (nude model-ing, may be prostituting etc).
What is the culprit's penalty hereafter in front of Allah.
waiting for reply. thanks in advance.

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.

If it is confirmed that these two girls had committed what is attributed to them in the question, then there is no doubt that they had committed a grave sin, however, this man killing them is also a great sin that is even graver than what the girls committed. Even if we presume that the girls committed what necessitates being killed, and this is most unlikely because there are some conditions that must be met and that are difficult to meet, then implementing the Hadd (i.e. the corporal punishment determined by the Islamic Law) is something that only the Muslim ruler or his deputy can do, as we discussed in Fatwa 84227; individuals cannot implement the Hadd.

However, we do not know a particular punishment for the killer in the Hereafter; his matter is in the Hands of Allaah, if He wishes, He punishes him, and if He wishes, He forgives him. Furthermore, the guardians of the two killed girls have the right to ask for legal retribution or accept the Diyah, or forgive the killer; for more benefit, please refer to Fataawa 119323 and 87847.

Allaah Knows best.