Think taller vs. shorter. It just is. And when I say "taller" it is in reference to someone else. My 5'2" wife is taller than my 7-year-old daughter. I'm taller than both of them. My wife is shorter than me. My daughter is shorter than my wife. So taller is descriptive of me but only in relationship someone else.
Same with sempai and kohai. Sempai "started before" Kohai. Kohai "started after" Sempai. Nothing more, nothing less.
That said there is a separate discussion of what makes a "good sempai" as well as what makes a "good kohai". And that will vary from context to context. Some sempai have no desire to help out, just train, and can even be a royal PITA. They're still sempai. And in some contexts you might be "expected" to listen to their horrid advice simply because they're sempai. If that's the case, you smile, listen, nod your head and say "hai", then go do whatever it is that is expected of you. In other contexts the relationships may be more "democratic" if you will with the implied "authority of experience" less in evidence. So.... Just pay attention to your context and try to get along. It will vary depending on where you are.
Since I'm taller than my wife (back to my original analogy) I try to take care of putting the dry dishes away that go into the high shelves. As a "good" taller person I realize it is easier for me to do it than for her. As a "good" shorter person my wife will herself put away the other dishes in the lower shelves knowing I'm going to take care of the dishes up high allowing me to get my thing done.
But some days when I fail in my duties I wander into the kitchen and find all the dishes put away. I failed my duty as the "taller person". And if you ask my wife I fail my "taller duties" a bit too frequently...