Are the two questions connected in your mind?
And how are you defining success?
I love my siblings. We get on great but don't see loads of each other.
Successful? No idea and have never thought to compare. And even if I did want to compare, it would be a mix I suppose. Financial, property, job, work/life balance, relationships, friendships, talents, skills, home life, life experiences, health, satisfaction with life - there are so many ways to define 'success'. And it changes through life as well.
I wouldn't ever think to compare my 'success' with siblings though. I want them to be as happy as possible and am happy for them when good things happen. I appreciate that if you are envious of the benefits your sister had, it would make it harder for you to be happy for her. But those things aren't her fault. And we aren't defined soley by our childhoods. Bad stuff can happen to anyone. Good stuff can happen to people who had a bad start.