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.
First of all, we console you for the death of your sister and we ask Allaah to forgive and have mercy upon her.
It is the court that should judge on this issue; if it is confirmed that the husband is the killer, then he deserves legal retribution; for more benefit, please refer to Fataawa 85485 and 82479.
However, you have the right to accept the Diyah (blood money) instead of legal retribution.
Allaah may punish him (the killer) in the grave because of this crime, but we are not aware of any evidence which states a specific punishment for a killer. As regards the Hereafter, Allaah clarified the punishment for deliberate killing; Allaah Says (what means): {But whoever kills a believer intentionally — his recompense is Hell, wherein he will abide eternally, and Allaah has become angry with him and has cursed him and has prepared for him a great punishment.}[Quran 4: 93]
For more benefit, please refer to Fatwa 87847.
As regards the money that you had given to that man, then you were not Islamicly obliged to do so, but since this is a custom and that man had already possessed it, then you have no right to ask for it. If we presume that it was a gift which was intended for the continuity of the marital relationship (between him and your sister), and it stayed with him for this entire period, then the objective is achieved, and it is not permissible to take back this gift.
As regards spending on the son and daughter, then it is their father who is obliged to spend on them if he has money, and you may ask for this at the Islamic court.
Allaah Knows best.