The sister alliance was dissolved. If they missed your invite, PM Wrily, because I REALLY doubt they actually meant to kick you, it's just that shifting a lot of members over to one alliance can be a bit busy.
Unless they explicitly told you they were kicking you, then nevermind, but you seem pretty active both in general and on the alliance forums, so it'd be odd if that was the case and you weren't told why.
This also isn't a gold user exclusive alliance. It's not expensive to play this game if you're active, especially with all of the gold spenders who will practically hand out gold for a small amount of resources.