Writing out of frustration

Reva • Mother of four children in a blended family, trying for #5. I`m an RN trying to balance family and career.
So let me preface this by saying that I love my step son to death and I have been the only stable female in his life. 
My step-son smashed his finger and the nail bed was damaged. I'm an RN and have years of wound care experience. I have been treating his wound and it's coming along just fine. He went to his mothers for the weekend. He sees his mother every other weekend and this just started a few months ago. She text my husband and bad a huge deal about it and says that my step son claimed my daughter smashed it. My hubby laughed it off and text back saying it wasn't my daughter. We got him back on Sunday then in Monday she writes hubby again saying it looks awful and she's worried about it. Then my hubby starts questioning me about it like I'm not doing enough for him! 
I'm pissed because his ex, a meth addict by the way, basically says it's not being treated well enough and makes hubby start questioning my nursing and parenting skills. 
I have also been in charge of this little boys medical care from day one, I've taken him to get caught up on shots, had tubes put in his ears and even put him on my insurance and haven't received one cent from his ex. 
I'm about ready to say "you all think you know better than a trained professional, then you all can care and pay for care for him!!!"
This lady frustrates me so bad and I am getting to the point that I feel a tremendous amount of tension. I buy him everything he needs, I'm there every day bathing him, feeding him, playing with him and it's like she has no clue how hard it is to raise kids let alone someone else's. I don't know what to do.