"Life Isn't Fair"

The divorce rate in America continues to climb. This probably comes as no shock to most of us but what about the children; the Biological (Bio) and Step children?
When you accept/offer a marriage proposal you take your mate with all their baggage, right?
What about the rest of your family? Will they? Should they?

I'm fully aware that no two situations are exactly the same and without enough detail you can't come to a logical conclusion but still I ask, "Is my mother obligated to buy her Step Grandchildren what she buys her Bio Grandchildren?"
I'm sure we've all heard the famous quote, "Life isn't fair". In my opinion the is the most candid quote ever but even with that said, shouldn't we still try to be fair? Is that not the right thing to do?

My younger sister recently married a man who had already had child and had one on the way which was a result of his prior relationship. My sister and her husband then had twins about two months after the baby on the way came. To date, there is a total of four children.
My younger sister is one of the most loving people I know. She has accepted his kids as her own.
Her love for them is open and she does for them whole heartily. If you saw them interact as a family you might even assume she was the mother of all four; triplets and six year old.
Her Step Children have decided on their own to call her mom despite their Bio mom's envy.
However, when birthdays and holidays come around is our family obligated to buy gifts or even buy a gift for her Step Children?
My sister's twins and her youngest step daughter are all young enough to use high chairs, hence the reference to them being referred to as triplets above.
For Nico's and JoJo's (the twins') birthday my mother bought them high chairs but she did not buy one for her Step Grandchild. Was she obligated to?
My mother asked me, "Should I have bought her a high chair?"
I was honest, "If it isn't in your heart then no you shouldn't feel obligated."
My mother than went on to say, "It wasn't in my heart or my budget! I know she's and innocent child but she has a Grandmother that will never buy my Grandson's anything."
I guess this is my mother's way of trying to make life fair, at least for the twins...!?!?!?!?!?!?!?!?

When I told my oldest sister about the situation she commented, "B (the youngest Step Daughter) is Cocoa's (my younger sister's) husband's favorite and B gets attention from her dad that the twins just do not get. I resent B and him for it regardless if it is wrong and I know I shouldn't, but I do. I can buy the twins a million high chairs but love is priceless especially your fathers."
Which is true, love is priceless. I nodded and my older sister continued, "If J (the older Step Daughter) was in the twins age group I'd buy her and the twins a high chair just because because their dad treats J like he treats the twins."
Don't get my wrong my sister's husband isn't the poster child for Messed Up Dads of America but because Life Isn't Fair he has his favorite. It is something that happens in many families, sometimes parents just have blatant favorites and it is something kids either deal with rationally or irrationally. It has been this way since the beginning of time and it will continue to be so. In this case my sister's hubby favorite just happens to be B. No matter what anyone says or what sense we try to knock into him B just happens to be his child of choice.
However, his favoritism for B effects the heart of those surrounding them looking in. Most of the twins' Bio relatives are not adamant about giving to my sister's Step Children because of her hubby's unfair behavior.
I know we are all entitled to our feelings and opinions but in an effort to make life fair and preserve the innocence of the children and although you weren't even asked to accept them in your life should you treat them the same way you treat your bio nieces/nephew/Grandchildren and so on?