Nymh...I've been doing research on this all day given our situation, I'm going to copy paste some Custody Info I found today on a court FAQ page...I hope this is helpful to you...

13) What impact does a parent's new spouse, live-in companion or other person sharing the home have on a custody decision?

Where another person will come in contact with or influence the child by reason of a remarriage or otherwise in a child's or parent's living situation, and there is a basis for concern about the stability of the child's environment, the mental condition and character of that other person are relevant in an initial custody or modification proceeding. The third party may be joined as an additional party to the case, and mental and other examinations may be ordered so that the court is fully informed. Where the third person is shown to have a history of child abuse, violence, substance abuse, or conviction of a serious crime, this will be considered by the court in awarding or modifying custody.

19) Once custody has been awarded, under what circumstances may it be modified?

Along with best interests of the child, stability and continuity in the child's custodial and environmental relationships are key concepts in Illinois custody law. As a matter of policy, the finality of a custody decision is considered more important than a redetermination of which parent should be the custodian. There is a strong legal presumption in favor of the existing custody arrangement. To overcome the presumption, one of two facts must be established: the custodial parent must be adjudicated unfit to retain custody, or a change of circumstances since entry of the previous custody order must be shown which directly affects the child. A mere finding that modification would be in the child's best interest is insufficient. Facts and circumstances which existed and were known to the parties prior to or at the time of entry of judgment, or a change of circumstances in the life of the non-custodial parent alone, are insufficient to support a change in custody.
Custody may be modified only upon a showing by clear and convincing evidence, based upon facts that have arisen since the prior judgment or that were unknown to the court at the time of entry of the prior judgment, that a change has occurred in the circumstances of the child or his custodian, or in the case of a joint custody arrangement that a change has occurred in the circumstances of the child or either or both parties having custody, and (b) that the modification is necessary to serve the best interest of the child. These criteria apply in any proceeding for change of custody, including a petition to change from joint to sole custody and where both parents have moved for modification.

24) Will a custodial parent's misconduct lead to custody modification?

Indulgence in moral indiscretions alone is not grounds for a change of custody where the children are leading a normal life. Where actions of a custodial parent seriously endanger the mental or emotional health of the child, however, the harm, if any, which might be caused by a change of environment to the custody is outweighed by the advantages of that change to the child. Recent cases suggest that, although the trial court should give due consideration to the custodial parent's living arrangement and its effect on the well-being of the children in determining whether a change in custody is warranted, conjugal cohabitation of a custodial parent in the child's presence with a person of the opposite sex does not, per se, require a change in custody of minor children. A past history of living with a person to whom one is not married will not foreclose custody.

My aunt has the same problem as yours. Good thing i saw some good suggestions here. It might be really helpful. - KSA Kosher