I know when I applied for the fiance visa my fiance could not visit. Does this hold true when we apply for his parents and siblings in a few years? Will his sister's not be able to visit while we wait 10 plus years for them to be able to move here?

It is up to the consulate if they will issue a visitors visa or not.

NOTE: A citizen filing for parents get an imediate visa number, parents like spouses qualify for the unlimited visa class. Siblings on the other hand are the lowest priority (F4) this can take decades to get a visa number assigned, it will be quicker for the parents to petition for the siblings once the parents are LPR or better yet Naturalized.

SEE: http://travel.state.gov/visa/immigrants/ty...types_1306.html