Sibling help Legal or illegal?

[Steven Hershey]

My wife and I are currently looking for our first rental property! I was wondering if it is legal to have a sibling do a mortgage application with me and my wife as a "co-borrower"  as a primary resident and use that to our advantage for lower rates and down payments for a rental. I would think this is ok because what if you bought a house and then rented it out after a few years? is there a year amount restriction?

our closing costs are about 5k with a 20% down payment @ 4.6% interest on a property were about to put an offer on. 

let me know BP! thanks