Corrections in red.  Stylistics in blue.

Is an explanation really required?

My beloved wife and I have two beautiful children -- two girls aged four-and-a-half, and eight.  And we wish to have one more (or two). And we want to take a child from an orphanage. Most of our friends and relatives misunderstand us. "Why do you need this problems? More money spent, more time spent…,"  they say. And what I can tell these people? How I can explain our decision to people who do not understand that on their own?  Of cause, that's a rhetorical question. We are not about to explain our decision. We are fully confident in its rightness/correctness, and in the and necessity of this step at least for one person – for child who will find a new family in our home. 

A sister of my wife is living in Sweden. That's where, she has told us, there are no orphanages. Foster homes for children are found very quickly. But only in the region of Zaporozhe alone -- a region of the Ukraine with a population of about 1,780,000 people) there are about 2,000 orphans.
So, as of today we started to prepare documents, and we are hoping that our daughters will have a sister by next summer.

Best wishes, and my sincere admiration for you and your family.