Jem
I don't know how my parents could have stopped problems, although I don't think they helped. They tended to stop in awe at my db doing things at a young age, when looking back he just had the opportunity at a younger age-for example we got our first computer when I was 10 and he was 6yo. They used to wax lyrical about how amazing it was to see a 6yo competant on the computer. Made me feel there was no point doing it, so I didn't try. Now, of course, my dc had surpassed what he could do age 6yo before they were 3yo, and I can see it isn't ability, it was opportunity. But I think the amazement was a natural one that we all have when we see our tiny children doing things we didn't do.
It also was pretty much everything I did too. He wanted to do it, so started younger, it went with gifts too. I can now, with my own dc, see how that happens with 3dc. The older one gets it, it's special, so you say to the middle you have to wait until the same age, but then it feels unfair that dc3 is the only one without it.
I think I would worry more because your older one has just started. Then they will feel that they're both older and started sooner that they should be better. If they started at the same time, or the older one had more of a head start I would imagine it could be less of an issue.
If you do start dc2, then I would say try not to compare. Never say "dc2 is doing so well because they are only X age". Because that knocks dc1-because they can't compete. And don't say "dc1 is better because they're older" either. Again something that dc1 can't do anything about-or dc2 for that matter.
Also if one starts the practice, the other can't start until the other has finished unless they're working at something together. I had huge problems in that I would start practicing, and that would remind db, and he would start, which did make practices hard when you're listening for tune. I was called mean for asking if he could wait until I'd finished, which meant I ended up restricting practicing to when he wasn't around, and that wasn't that often.
I think the big thing is to make sure they each have their own thing. It is quite natural for the younger to want whatever the older has-but to me it's important that you do have your own niche in your family.
You could discuss it in a casual was with dc1. Not asking "do you mind" but throwing the idea out and seeing how they react. Not to give them the decision, and you phrase it so you're not asking them, but to see how carefully you need to tread.
If you say "You're enjoying the violin so much, do you think dc2 will want to play soon?" and they go silent, or get upset, you need to be much more careful than if they say "we can play together. That'll be fun."
Sometimes with my dc I have been very pleased when I've mentioned to dc1 that dc2 is thinking about joining , and dc1's reaction has been along the lines of "good, I can't wait."
But I do protect dc1 in that she has her thing that I won't let dc2 do. I'll let her do a similar class, but not the same. Neither of them know this, I've just said to dc2 when she asked that she couldn't do it due to another committment at present. And same with dc2 and dc3.
I think it's important that they do have the space to be themselves, and not just X's sibling-which is important for the younger as well as the older.