I've been theorizing for a while back here, and I've been family shipping background ponies.
From left to right: Stormfeather, Apple Stars, Princess Erroria, Cloud Kicker, and Goldengrape
Here's my obscure animation evidence, because no one cares. First, the theory as follows: Stormfeather and Apple Stars are the parents of Cloud Kicker, Goldengrape, and Erroria. More family to come. I'm also theorizing, due to coloring, that Flitter and Cloudchaser are also related. Perhaps cousins. But I thought of that after I finished drawing this. Sea Swirl's family will come along soon, and that picture is a clusterbuck.
Cloud Kicker and Erroria are obviously sisters as seen in Sisterhooves Social
There's a scene during A Friend in Deed during the Smile Song where we see Erroria and Apple Stars standing just inside a window together. Whenever I see children with adults I assume they're related when they're alone like that.
We see Goldengrape and Erroria together on Nightmare Night and it looks like she's talking to him.
Stormfeather just has similar colors and is a pegasus.