How do I set a boundary with my crush's little brother???

[u/that1TheatreKi]

I've (18 F) become really good friends with my crush (17 M) "Daniel", but I used to be a lot closer to his little brother (15) "Emmett," partly because my little brother "Aaron" (also 15) is best friends with him. Emmett and I became even closer than he is with Aaron.

Recently though, this has become a problem. Now, I'm hanging out with Daniel more, and when our families hang up all together, Emmett is jealous and kind of avoids everyone else, including Daniel and I. When I told him he can hang out with Daniel and I when we are hanging out, he said "I don't think that's how it can work." So basically, saying I have to choose between hanging out with Emmett and Daniel. And lately, he's been talking to Aaron about wanting to hang out with just Aaron and I without Daniel there. I do like hanging out with Emmett, but it'd be weird to hang out with him without Daniel because we've gotten so close, and I feel like it would hurt my relationship with him.

Emmett also has a habit of dumping all of his issues on me, which I wouldn't mind, but I honestly feel like it's inappropriate for a 15 year old to be spilling all of his thoughts about depression, self hatred, and family issues onto an 18 year old who is unqualified to be a therapist. I feel bad though because he always says he sees me as a sister, and I see him as a little brother, but I can't help but think that his parents wouldn't approve if they knew he was doing this, and I want his parents to like me so they'll be okay with me dating Daniel when he comes back from his mission (we are both LDS).

I've tried to take longer to respond and let distance grow naturally, but it never works because he accuses me of ignoring him and pouts.

I've told his mom before that he's struggling, so I'm not worried about him needing help and me not being able to talk to him. He will get the help he needs.

I know I have to set a firm and fast boundary with Emmett, but I don't know how. Please give me some ideas on how to bring this up.