Dealing with foster care jealousy

Hi parents! So I'm looking for some advice. I haven't posted on a while. Me and my husband have a son who's 17 now and adopted from the foster system. A lot of you guys followed our journey with him because originally my husband agreed to house him without consulting me. We said no violent children. He was in a mental hospital for 6 months for trying to burn his last families house down but he was severely abused in that home. Physically by mom and dad and foster brothers but his foster dad's and foster brothers all raped him. That's what lead him to snapping.

He's been an amazing addition to our lives. I always planned to foster. Not adopt but we can't imagine our lives without him. Me and my husband thought about fostering again. We asked him and told him we think he would be a great big brother to them. Even after he said he was okay with it we waited quite a while. We just got a 9 year old foster daughter. Her mom was a drug addict and she was touched by her step dad. There's definitely some jealousy and it's caused some regression on our son's side. He sees a therapist twice a week. He hasn't tried to interact with her and has been very clingy lately. He's also going through a break up with his bf so I think it's make the clinginess worse. Of course we are gentle with him. A little conflicted happened when he had a nightmare and we were trying to calm him down. He wears goodnites because he has accidents at night. He went to the bathroom to change and our foster daughter had gotten up to be and walked in on him. At breakfast she said her 5 years old brother wears those diapers. He got upset and left the table angrily. I've spoken to him and he said he was sorry if he's being a brat. I just reassured him that he's not being replaced. We are trying to help her too. How can I help make this easier for both of them.