1. Good, I'd recommend a big one to start of with (or medium at least)
2. Sure, I'll contact you via In-game Private Messaging
3.
4. Yes, purchase land after buying Infra
5. x stands for a random number. Basically, it means that your total infra should be 59.99, 199.99 or 299.99 etc.
6. Unless your sister lives in another place with a different IP, you shouldn't trade with her as it would break the game's rules. In CN there is a system called Trade Circles when 6 people all trade with one another and gain Team Colour bonuses and Bonus Resources. As said before, your Alliance should do this for you or you can organise one yourself on these Boards.