Since your daughter and her BF live together ---only one of them can claim the bio-child/new baby they have on a tax return. If BF got the advance CTC payments and stimulus for his other child---who he is NOT claiming on a 2021 return (right?) --- he has to enter the advance CTC payments he received AND the stimulus he received on his own 2021 tax return. If he claims the new baby--he does not get another full stimulus payment for her. He would only get a partial CTC payment for the new baby because he already got CTC between July and December.
If your daughter files a 2021 tax return she can get the stimulus and CTC for the new baby. You say your daughter also has a son----so she can claim her son too? Or is her son being claimed by a different parent for 2021? If she claims her son and the new baby she gets the other half of the CTC for the son too.
Oh... and you asked---yes, the BF's ex can still get the full CTC and stimulus when she claims the child on her 2021 return.
**Disclaimer: Every effort has been made to offer the most correct information possible. The poster disclaims any legal responsibility for the accuracy of the information that is contained in this post.**
