Hi FamilyGuru -
You would likely have to form a partnership of some sort or have some other binding agreement if you all want to share ownership equally. The partnership would own the house and your parents would pay rent to it. Or you may be able to set up a trust of some type that would buy the home and rent it to your parents.
It's possible that one of you could simply buy the home in his or her own name, and have the siblings contribute toward the mortgage payments or your parents' monthly rent. However, without a formal agreement that money cannot be counted as income for purposes of qualifying for the mortgage, nor would the siblings have a shared ownership interest in the property or be able to realize any tax benefits from their contributions. There may be other complications as well.
When buying a home with the intention to rent it, even to your parents, the home is considered an investment property so you'll pay a higher rate than you would on a home purchased for your own occupancy. You also need to be up front with the lender about your plans for the property -- you can get into legal trouble if you mislead a lender about how you plan to finance and use a property. You also need to charge your parents a rent that is reasonably close to market rates, or the difference could be regarded as a gift by the IRS for tax purposes.
This is a fairly complicated situation where you really need to meet with a financial advisor who can explain the ins and outs of the situation and go over the various options available to you, as well as making sure you're doing everything properly from a legal and lending standpoint. Good luck!