Single Parenting Support Group
Today being raised by a single parent is not uncommon. About three in ten children live in a single parent home. The most common type of single parent home is one with only a mother. However, single father homes are the fastest growing type of family situation; 60% growth in the last ten years alone.
I wanted to say something a preacher friend of mine once said "Even the greatest of men are tempted and fall to sin."
It just means no matter what we all made mistakes.
Your brother in law is probably angry over the divorce for many reasons, and will be judgmental with a chip on his shoulder for a while (and some never get over it) but divorce is NOT evil.
The bible even allows provisions for it. I don't believe a minute we were doomed to spend eternity in heaven with a man who made our life here hell. a good reference if you are Christian http://www.bible.com/bibleanswers_result.php?id=151
As for how to deal with the brother in law? I hate to say stop giving him or his family things. I know you love them despite everything. You may want to limit how much and how you give.
There are plenty of people who would appreciate and graciously accept those gifts.
The things you want your niece to have. Just remember, you aren't doing it for your brother in law. Think of the smile that girl will get don't let his bitterness lead to your unhappiness.