bestfriend might neglect me :(

so my guy bestfriend is starting to see this girl, who i don’t personally know but haven’t heard the best things about. He likes her and she likes him and they are having a sleepover soon and are supposedly gonna ‘ go for it ‘. I’m trying to be okay with this, will he neglect me after and push me to the side. We are like best friends and basically siblings. Lately I have noticed he’s a bit distant tho. I don’t know what to feel, he’s mentally un-stable so idk weather this is good for him but the things she dose and can provide is not good for him. I DONT KNOW WHAT TO DO OR FEEL! please help !!! THANK YOU