Technically IF the children ARE qualifying children of both the parent and the 19 year old then the tie breaker rules come into play ... so the 19 year old can only claim the siblings IF the mother does not AND his income is larger than the mother's ... which negates the real reason you are asking this question ...
Tiebreaker rules.
To determine which person can treat the child as a qualifying child to claim these five tax benefits, the following tiebreaker rules apply.
If only one of the persons is the child's parent, the child is treated as the qualifying child of the parent.
If the parents file a joint return together and can claim the child as a qualifying child, the child is treated as the qualifying child of the parents.
If the parents don't file a joint return together but both parents claim the child as a qualifying child, the IRS will treat the child as the qualifying child of the parent with whom the child lived for the longer period of time during the year. If the child lived with each parent for the same amount of time, the IRS will treat the child as the qualifying child of the parent who had the higher adjusted gross income (AGI) for the year.
If no parent can claim the child as a qualifying child, the child is treated as the qualifying child of the person who had the highest AGI for the year.
If a parent can claim the child as a qualifying child but no parent does so claim the child, the child is treated as the qualifying child of the person who had the highest AGI for the year, but only if that person's AGI is higher than the highest AGI of any of the child's parents who can claim the child.
Subject to these tiebreaker rules, you and the other person may be able to choose which of you claims the child as a qualifying child.
