Someone please tell me if I'm wrong..

Back story: Due to the lies of my ex husband whom was the custodial parent of my children (we had joint custody) my sister is fostering them because I finally got the police and DCFS to listen to me about him abusing them. He has 2 pending aggravated battery charges for this reason.

In the time of my husbands care, my son now 13 was molested by one of his girlfriends nephews. With that, since being in our care (I say our because I have my children most of the time but TECHNICALLY with dcfs involved, they reside with my sister until all the court stuff is settled.

My son is showing regressive behavior (hes in therapy) where he is having bowel movements in his underwear. Due to the shame, hes throwing them away because hes been punished by both my ex and recently my sister for it.

Today is his birthday party. He received money from my parents and my brother yesterday because they cant make it to the party. My sister has decided that since he messed up her son's deodorant and has worn his underwear (which is in my mind impossible, because my son is 13 and her son is 9 and scrawny) that and she has zero proof hes actually worn them, shes taking his birthday money he gets and buying her son deodorant and underwear. I'm highly pissed. Idk what I should do. The me in me wants to knock her the fuck out. But the good in me just says to let it flow and then when all of this is said and done, just cut ties. Am I being petty, am I wrong, is she wrong?