The song is from season 2 episode 25 of MLP:FiM. Reprise: https://youtu.be/07zp6EMIRdQ | ♫ Lyrics for the song:
When I was just a filly, I found it rather silly To see how many other ponies I could meet I had my books to read, didn't know that I would ever need Other ponies to make my life complete
But there was one colt that I cared for I knew he would be there for me
My big brother, best friend forever! Like two peas in a pod, we did everything together
He taught me how to fly a kite (Best friend forever!) We never had a single fight (We did everything together!) We shared our hopes, we shared our dreams I miss him more than I realized It seems...
[Rest of main six]
Your big brother, best friend forever Like two peas in a pod, you did everything together
And though he's, oh, so far away I hoped that he would stay My big brother best friend Forever... Forever...