I'm not sure why there is such a need to tell people what they must or must not do.
It would be nice for everyone to love and treat people the same. Family, or not. However, everyone is entitled to do what they want, and different situations and factors can be the cause which makes this difficult at times. Consequences can come into play also, if people feel disrespected in any way.
Just a side note: We should remember that if we are receiving total disrespect from anyone, including an extended family member, then that might make it extremely difficult to treat them as part of the family.
So let's just say that things are fairly amicable "on the whole" in the OPs case. Well they were before her parents found out about her ongoing deceit and lack of regard for them.
As I have said earlier in my posts, I, personally, do not think the parents giving money to the GC and not the SC was wrong, but I do not agree with not including them in the Xmas boxes. It would be nice to give them something. However, whatever they decide to do is their perogative.
I also do not agree with the OPs deceitful way of handling the parents gift. We see the consequences came into play on both sides because each side felt very disrespected.
It is indeed selfish of the OP to "expect" her parents to treat the SC exactly the same as the GC. Re-distributing the parents gift to the GC because they did not follow her request was very selfish and disrespectful. She did what she wanted to make herself feel better about the situation. She had other options but she chose to go against her parents wishes. It doesn't matter why she did it. It was totally out of order, in my opinion.
When we start expecting people to do things, and we disregard their decisions and control what they do without their knowledge, it becomes a selfish act, to say the least.
I believe anyone who believes this is OK and there are very many on this thread who do, are equally as selfish and disrespectful.