Hi H.,
I think people today are very lonely. They do not have the comfort level or closeness with their sibblings to go to them for help or emotional support. Especially if they're different genders.
I think the fact that I slept with my sisters is a huge contributing factor that we do not fit into that mold. We are so close, we are all married with our own children in different states and we still see each other yearly with our children. We speak regularly on the phone and email daily.
It's a shame that children aren't allowed to be closer. In other countries, they all sleep in the same room and they are so close and comfortable with touch and physical affection. I know people who can't even HUG. So sad!
I would recommend, if you do put them together, do it during spring break, make them look forward to it, they will keep each other up a lot at first so it gives them some time to acclimate to each other.
Because they are different gender, I would only do it for a couple of years.
It will also teach them to share, to lean on each other, and who likes to sleep in a big room all alone? I didn't!
I foresee the SAME issues with my kids, they're 11 months appart, she's 18 months now and he's 7 months.
I hope whatever decison you make works.