The back story: I've never met FI family (he rarely see them), but his sister friended me on facebook immeadiately after we got engaged and I sent her an email introducing myself and our intent to have a wedding this coming spring. She seemed excited and said that she'd love to come. A month later, my FI sent a group facebook message to his parents asking, in short, who wants to come to our wedding (I know, super rude, I did not approve). But his mother replied and said that she's planning on coming as well. We live a long drive and expensive plane ride from where his family lives.
Here's the question part: Can I send his sister and mother informal save the dates now via email with hotel and area info, or should I wait til the formal ones go out at the 6 month mark?