I do not think it has specific way to address.

TO whoom it may concern;

I .......... would like to invite my parent:............................ During their vist to me they will stay with me and I will bear all the accomodation cost.

You need a bank balance to show you have fund for their accomodation. You may or may not like to specify the date of their living with you.

Just make is simple and to the point. I do not think you need a fancy letter.