There are a few things you can do. First, you can (and probably should) contact SRS and relate your concerns about the situation -- both for the children and for your father. SRS can investigate child abuse and neglect complaints (they are anonymous). If SRS finds that there is concern, then they can take the children into protective custody and the District Attorney can file a complaint for a Child in Need of Care ("CINC" in Kansas). The alternative is that you could file a private CINC petition. But the burden of proof is high and the facts that you mention don't appear to come close to showing any kind of unfitness. Further, merely taking drugs and/or alcohol does not make a parent "unfit." Finally, even if the SRS finds that there is concern does not mean that custody is given to anyone else. SRS and the courts are required to assist parents in preserving their parental rights and in resolving the issues identified. But the only way that you can know what to do is to make an appointment with a qualified family lawyer, reviewing all the facts of which you have knowledge, to know what you can and should do.
You mention "the child," but it's unclear if you are referring to "[your] half-sister" or someone else or if your half-sister is in addition to the child.
One correction to the other response that must be made is that a mother does not automatically receive or have the right to sole custody if parents are unmarried -- in Kansas or in many other states. "Sole custody" is granted only when there is a parentage order or divorce order on file after the court finds that order appropriate and in the child's best interests. Otherwise, lacking an order from a court, "presumed" mothers and fathers (which would appear to include your father in this case since he and she appear to recognize him as the father) have equal rights to decision-making and "physical custody" of the child.
This response does not constitute legal advice and does not create an attorney-client relationship. I am licensed to practice only in Kansas. Seek legal advice from an attorney in your state or the state in which your legal claim exists.
You can always file a CHIPS complaint, and social services will investigate. But, if as you suspect, the investigation doesn't turn up anything illegal or "dangerous" on the mother, then nothing will come of it. Bigger concern here - your father - is he married to the mother? Just because he is the biological parent to the 4 year old does NOT mean he is a legal parent. An unmarried woman is automatically given SOLE legal and physical custody of the child at birth. If they aren't married, and your father never went to court to obtain some kind of custody rights, then he cannot do anything even if he wishes to - he has no legal rights to the child. You father is free to move out whenever he wishes - unless he's declared incompetent and you are named as his guardian, you can't "take custody" of him. And, you won't get custody of the little girl unless you can PROVE the mother is placing the child in danger, and only then could you get custody if you launched a third-party custody action.
We can be reached at 507.334.0155. Our web address is: www. corbin-law-office.com This answer is not to be considered a response to a specific legal issue in a specific jurisdiction - it is to be considered only a general response to a hypothetical scenario posed by the questioner. For specific legal advice, please consult with a licensed attorney in your jurisdiction.
Sign up to receive a 3-part series of useful information and advice about child custody law.