I would suggest taking them each out separately and just asking if there's anything going on.
My and younger sibling were like that very suddenly, mostly stemming from a couple of incidents that hurt me very badly, but my parents refused to admit they could be an issue at all. I tried talking about it, and was told I couldn't possibly mind, which very much added up to the feeling I was worth less.
If they'd listened and dealt with, I think things could have been very different. Unfortunately they chose to take the easy option which was basically give way to the (admitted by them) more awkward one.
After that, I think they're young enough to say things along the lines of, if you get on we can .
Don't force them to do "family time" and don't always give way to the younger, or have the younger always following the older. Make sure they do different interests, and have their own space. Encourage them to be themselves and not just want to beat the other one.
And make sure punishments are fair-which isn't always equal. Grounding them both isn't equal if one is due to go out with friends tomorrow and the other wouldn't go out at weekends/evening if you paid them.
But most of all listen to them. Not when they're going "it's not fair, he had two grains more than me" and "she pulled a face at me". But when it's them talking separately, not in the heat of the moment. If they say something has upset them, then it probably has, and telling them they can't possibly be upset because it's too small/they didn't say anything at the time etc. Talk it through, let them know that sometimes you can't be equal, that sometimes they will disagree with your decisions, but at the same time, they can come and talk to you and you'll take it seriously. Let them feel that they are as important.