>Was Margaret ever openly against her sister?

Yes, she showed out many times at the annoyance of the RF. But, the RF was so small that they didn't have backup options so it's not like the Queen could or wanted to replace her as a working royal. Now the core family is large and they have options.

>Did Charles's siblings hate him for being born first?

Stop the drama! There is no proof that Harry hates his family or that Charles siblings hate him. But we do know that Charles and Andrew have never gotten along. Charles made it clear that Andrew's kids would never be considered core members of the family which made Andrew angry.

>It's like they are advertising that you should never have siblings because they see them as some sort of competition.

It all depends how they are raised. I assume, though don't know, that there was some parental gap between Diana's death and Harry's adulthood. It's said that William, a typical teenager, ignored Harry while at school. So these boys are in boarding school, their doting mother is dead, and all they have are teachers and a seemingly distant father. I can see how he could feel like an afterthought.

But, Harry is still a rich, titled, privileged man so these are the tears and drama of a 1%er.