"stays the night there a lot"
How much is a lot? If it is more than half the nights in the year, then the child "lives with" his great aunt & uncle, for tax purposes, and they are allowed to claim him
If that's not the case, the tie breaker rules are not relevant because the child lives with you, his parent, and you can claim him.
This situation happens all he time and the IRS has a procedure for resolving it. It won't cost you anything other than bad feelings with family.
If someone else claimed your
child inappropriately, and if they file first, your return will be rejected if
e-filed. You would then need to file a return on paper, claiming the
child as appropriate. The IRS will process your return and send you
your refund, in the normal time. Shortly (up to a year)
thereafter, you'll receive a letter from the IRS, stating that your
child was claimed on another return. It will tell you that if you made a mistake
to file an amended return and if you didn't make a mistake to do nothing. The
other party will get the same letter you did. If one of you doesn't file an
amended return, unclaiming the child, the next letter, from the IRS, will
require you to provide proof. Be sure to reply in a timely manner.
Winner gets the tax benefits;
loser gets to pay the IRS back with penalties and interest. The
custodial parent almost always wins.
https://www.thebalance.com/claiming-same-dependent-audit-risk-3193030
