Don't let little sister always do what big sister does. Give her space. Just because big sister decides to do netball (or whatever) doesn't mean that little sister starts it too. That way big sister can carve a niche out for herself without feeling little sister is snapping at her heels.
If they start something together, or little sister just after big sister, don't exclaim how amazing it is that little sister can do it when she is only X years old. The younger ones do gain skills the older ones have simply because they have the opportunity.
Don't encourage the older by knocking down the younger. It won't ring true to the older and will encourage the competition. I'm not sure saying "I don't compare you" totally helps because there is always a slight humiliated feeling that others (even with no evidence) are saying "#2 is beating her big sister".
Try and encourage them into different groups when they go to things if possible. Don't push the younger up, or older down (in age groups) so they can be on their own.
If there is something that the older is better, don't push the younger to do it. Because actually the older can't win. If the younger isn't as good it's because they are however many years younger and will catch up (in the older one's eyes).
If the older one achieves something the younger one doesn't, don't justify why the younger one didn't get it. Just celebrate the older one's success (and same for the other way round)
These are all things I found trying growing up.
I was the middle one. Dc1 was all round bright. Good at everything regarded as academic. I knew I couldn't catch up with her. Dc3 was touted as a child genius. Everything he did was treated with amazement. I felt I wasn't able to compete with either. So I didn't try. I felt it was better to fail not having tried, rather than work hard and still not do as well. Because then I could always pretend I might have done as well if I'd worked.
I was very one sided. I could do maths and only maths. And actually I was far and away better at it than the other two. But it was never commented about. I remember talking with friends before we did our GCSEs and them saying how their parents would be pleased if they got good results and one turned to me and asked and I replied "If I've done as well as dc1 then they'll say "good, now you'll have to do as well as them at Alevels" but I probably won't do as well." And I thought to myself "and dc3 will do much better".
Actually I did better than both of them, but that never registered until adult years.
I was also majorly into a sport I was fairly good at, and I was musical, which the other two were definitely not. Dc1 wasn't interested, but dc3 had to do exactly what I did. Looking back, again I cn see that I was clearly better than dc3 from the beginning and all through, but it was never stated. He was always doing brilliantly considering he was so much younger than me... I think if they'd encouraged dc3 to do a different sport and instrument, I would have felt a lot less threatened.