It's always best to write about what you know. I think it's ok as long as your Russian characters are only secondary persons or if you have extensive understanding of Russian life, nuances and beliefs. You could seriously shoot yourself in the foot if actual Russians don't find your characters believable.
Of course it's OK. However it may be more difficult to write a good Russian character if you don't know anything about Russians. A character should be believable. If it's just something you are writing casually it won't matter much. But if you are doing serious writing you need to do some research, not just on your characters, but everything else.
Almost every single story includes at least someone not from the author's country of origin. Your dad is a misguided patriot that think you should only write about Americans when in reality you would only be limiting yourself.
It's best to write about what you know about best. I suggest if you want to create Russian characters then you may want to read up on Russia and their culture, mannerisms, etc. so you can more accurately portray one in your book.