I was in a relationship for 9years with a woman. She has two kids from a previous relationship. We now have a 4 year old daughter, her father is a blood relative to me. Before her birth my partner and I did the paperwork for a domestic partnership so I would have rights to our child. When she was born I was put on the birth certificate. My partner and I have since split up back in August 2011. Reason being she was addicted to prescription drugs and refused to seek help, she has now gotten help. I filed for custody of our daughter back in August. She is fighting me on this. When she left our home she left her two kids behind and I have been supporting them since. She is only fighting to get our daughter back. Her two kids don't want to live with her. What are my chances of getting custdoy of our daughter? And what rights do I have in regards to the other kids? What can I do in regards to them?

What are my chances of getting custdoy of our daughter? And what rights do I have in regards to the other kids? What can I do in regards to them?

The legal basis

§ 3101 (a)Notwithstanding any other provision of law, the court may grant reasonable visitation to a stepparent, if visitation by the stepparent is determined to be in the best interest of the minor child.
(2)"Stepparent" means a person who is a party to the marriage that is the subject of the proceeding, with respect to a minor child of the other party to the marriage.
§ 3040(a)Custody should be granted in the following order of preference according to the best interest of the child as provided in Sections 3011 and 3020:
(3)To any other person or persons deemed by the court to be suitable and able to provide adequate and proper care and guidance for the child.
§ 3042 (a)If a child is of sufficient age and capacity to reason so as to form an intelligent preference as to custody, the court shall consider and give due weight to the wishes of the child in making an order granting or modifying custody.

§ 7551 In a civil action or proceeding in which paternity is a relevant fact, the court may upon its own initiative or upon suggestion made by or on behalf of any person who is involved, and shall upon motion of any party to the action or proceeding made at a time so as not to delay the proceedings unduly, order the mother, child, and alleged father to submit to genetic tests.

§ 7573Except as provided in Sections 7575, 7576, and 7577, a completed voluntary declaration of paternity, as described in Section 7574, that has been filed with the Department of Child Support Services shall establish the paternity of a child and shall have the same force and effect as a judgment for paternity issued by a court of competent jurisdiction. The voluntary declaration of paternity shall be recognized as a basis for the establishment of an order for child custody, visitation, or child support.

§ 7574(6)A statement by the father that he has read and understands the written materials described in Section 7572, that he understands that by signing the voluntary declaration of paternity he is waiving his rights as described in the written materials, that he is the biological father of the child, and that he consents to the establishment of paternity by signing the voluntary declaration of paternity.

§ 7575 (b)(1)Notwithstanding Section 7573, if the court finds that the conclusions of all of the experts based upon the results of the genetic tests performed pursuant to Chapter 2 (commencing with Section 7550) are that the man who signed the voluntary declaration is not the father of the child, the court may set aside the voluntary declaration of paternity unless the court determines that denial of the action to set aside the voluntary declaration of paternity is in the best interest of the child,after consideration of all of the following factors:
A)The age of the child.

(B)The length of time since the execution of the voluntary declaration of paternity by the man who signed the voluntary declaration.

(C)The nature, duration, and quality of any relationship between the man who signed the voluntary declaration and the child, including the duration and frequency of any times during which the child and the man who signed the voluntary declaration resided in the same household or enjoyed a parent-child relationship.

(D)The request of the man who signed the voluntary declaration that the parent-child relationship continue.

(E)Notice by the biological father of the child that he does not oppose preservation of the relationship between the man who signed the voluntary declaration and the child.

(F)The benefit or detriment to the child in establishing the biological parentage of the child.

(G)Whether the conduct of the man who signed the voluntary declaration has impaired the ability to ascertain the identity of, or get support from, the biological father.

(H)Additional factors deemed by the court to be relevant to its determination of the best interest of the child.

So let me get this straight. You she and her two children lived together with your joint child. Then she moved out last August. When she moved she took your joint child but left her two children with you.

I think you stand a good chance of getting custody, especially if you can prove the drug problem. As to her two kids you currently have no legal standing, but given that you cared for and continue to support them, you should have no problem getting legal guardianship.