The permanence as a family unit of the existing or proposed custodial home Show the judge your plans for a stable family relationship should you be awarded custody. By outlining your proposed living situation and comparing it with that of the other parent you may be able to sway the judge to your side. This factor can particularly come into play if you are proposing to change custody after the divorce is over. If you have remarried, and live in your children's current school district, a judge may deem yours a more stable family situation than that of your former spouse. Another situation might be where your spouse wants one of the children but not all of them.
Courts are reluctant to separate siblings unless there is a very good reason for doing so.
If you have a plan to keep them together, your plan is more likely to be favored. Work with an experienced lawyer in your state.