All the advice about being fair, with money, time and energy is good (though this is hard as they get older and you do different things with them).
My other advice is- open up the channels of communication with both of them separately, so that if there are any issues or resentments, and there's been a couple over the years, they can vent them and you can explain your own decisions. Often children latch onto something as 'unfair' when your thought process is completely different. So, being honest, explaining your thinking works well.
I'd also say they may have times where they don't like each other very much. Mine were very close as toddlers, through the primary years, but then once the eldest got to 10, she just moved onto teenage/pre-teen type books and clothes and life, when the youngest still wanted to play dress up. There was a period of at least a year or two where the oldest disliked the younger. I think it's ok to dislike someone, but not to be mean to them or be actively horrible. So, she was told to keep out of her way, or take herself off to the bedroom, not roll eyes and be nasty.
I don't believe though in letting kids fight it out (literally) between them, I know some do, if they started bickering or being nasty, I'd send one one way and one the other way. I don't want to listen to that, thanks. If you don't like your sibling being downstairs, go upstairs, if you don't like them coming in your room, get a lock. I don't like mean/nasty behaviour, although with two teen girls there have been the odd outbreaks over the years!