I'm sure you'll get some good responses from breeders or others that have been through the sibling thing with their dogs. In my case I only have Ruby, but we see her sisters at her training class. I would have expected her to want to romp and frolic with them- nope. Like yours, she does well with other dogs, but if it's one of her sisters she quickly gets fluffed up and will snarl at them. All I can think of is that it's a hierarchy thing. Who's top dog of the originial pack and all.
As far as cues that it's not play. Ruby will immediately try to stand taller- almost on the tips of her toes. She will also prick her ears forward. And the definite tip off is that she will become very still when they are close- like she is waiting to pounce. When she plays or is relaxed she's almost in constant motion and no rigidness.
In the very beginning she played with her sister once. They were rambunctious but then I noticed that one kept pinning the other day and it quickly escalated into grabbing for each other's scruffs- they don't play together anymore.
My point is that they may just not want to be buddies. I know you can train them to behave themselves around each other, but I don't know if you'll actually get them to like each other.
Again, I'm sure you'll get someone to respond that has a lot more experience with this. I'll be interested in finding out too.