Okay so, I love Spideypool, I love Flashkaine and I love the idea of overprotective older sibling Peter Parker so ... here is a scenario I tought about.
*In the middle of a battle*
Peter: Flash listen, Deadpool is here, don't hit him! He's our ally and *sighs* ... my boyfriend.
Flash: ... I'm sorry, what was that?
Peter: I'm dating Wade Wilson, okay?
Flash: Deadpool? the deadpool? like, black and red, crazy as one can get, kill first ask questions later deadpool? You of all people went for that one?
Peter: We're working on the "kill first" aspect, okay? but yes, I'm dating him.
Flash: Okay, listen. I wasn't gonna say anything, especially not now, but given that you have revealed your terrible decision making skills right here, I'm feeling like it's a good moment to tell you this. And you don't get to kill me! Because your choice in dating is much more terrible than mine.
Peter: Flash, what are you talking about?
Flash: I'm in a... kind of... relationship, right now. With your brother.
Flash: It was a total accident. I was passing trough Houston to chase Carnage, and he was fighting Kaine, so that's how I met him-
Peter: KAINE? YOU'RE DATING KAINE?
Flash: Yeah but listen! I didn't like him at first, you know he's a difficult little shit, and I almost killed him when I lost control of the Symbiote, but then we got teleported to the microverse and yada yada, we fought together, almost died together and I noticed he's kind of cute, and after that we really hit it off you know what I mean?
Peter: No I don't! you pervert, you went after my baby brother?
Flash: Baby brot- Kaine is a grown adult! Biologically speaking he may as well be your twin! And like, it's not that serious okay, we're just fucking-
Flash: Okay, that- that came out wrong-
Wade: yes, itsy bitsy honey bun sugarcube?
Peter: I know we talked about starting to use rubber bullets and I love you for actually listening to me and accepting my opinion, but I need you to pass me a real gun right now. I need to shoot a baby brother snatcher in the dick-