I was the middle child - I have a brother that is 2 years older than me and a sister that is 2 years younger. In a way that gender makeup seemed MORE difficult for me.
My brother was older so I wasn't supposed to be "pesky". I was supposed to leave him alone so he could do boy stuff.
My sister was younger so I was supposed to be the 'big sister' and let her tag along and I had to let her play with my friends. Hello, Double Standard!!!!!!! On top of that we shared a room which I absolutely could not stand. I couldn't get away from her. To this day we are not close.
Along the same thing - I always seemed to be the one that got in trouble even though I was a classic mediator, trying to smooth it out for everyone. She would ALWAYS either take the side of my brother (cuz he was older and the boy) or my sister (cuz she's the baby).
In my experience as a middle kid it's like of like you just get sh*t from each side.
I am HUGE with validating feelings - I think that might have made a huge difference for me, if I hadn't been told to love my sister all the time and don't be a pain to my brother. I think if my mom would have just once looked at me and when "yah - that's kind of tough. I get it".
I was in all the activities my brother was in cuz it was easier to drive us to one place when we were all young. So I had to play soccer, be on swim team and take piano lessons (my brother is wicked talented musically, so this was the MOST torturous). What I really wanted was to do ballet. When finally my mom said I could take ballet - guess who HAD to be in my same class? You guessed it!!!!!! "oh... is that your little sister... she is soooooo cute".
So, they each should have their own activities. Don't make them do stuff they hate just cuz it's easier for you (I know - hard to avoid sometimes!).
HA! Can you tell, I'm 38 and STILL bitter!!!!!!!!!!