Uke and seme are maybe just sleeping together or possibly working out their relationship
Uke ends up taking care of relative's kid or a kid of family friend for some time(may have lost his parents or having family issues) (or maybe it's a student? not sure but is younger). Kid ends up getting attached to the uke and possibly falls in love with him. Kid is a teenager
Uke and seme into a fight or something.
Uke may have gotten upset and made to cry by the seme and the kid comforts him and maybe and gets too close and kid kisses him or something and confesses he loves the uke.
I think the confession is seen by the seme and he gets angry and but the uke doe reject the kids advances through his actions when seme does come between them and uke chooses the seme. Kid does realizes he can't win against the seme and gives up, but tells him, if he ever gets sick of the seme or breaks up he is there for him before leaving them alone to be together

Seme may see the kid as a brat or annoyance
Maybe uke sees the kid as what helped them realize their true feelings

I don't know if it's this but : dekichatta danshi

That ones isn't the one been looking for, thank you anyways

This sounds so familiar. I don't think this is the right title, but it's similar to the first volume of Kizuna. https://www.mangaupdates.com/series/4wbjqxk/kizuna

Ranmaru, the uke, is in long term relationship with Enjouji. Kai is Enjouji's younger half brother and has a huge crush on Ranmaru.

that is not the right one either, been meaning to give it a read for a while though >w< Thank you anyways

have you found it?

Sorry for the late reply
haven't found it yet