I've got a sibling problem that makes the "Cradle to Grave" title of this post slightly scary. My husband is one of eight siblings, and I've come to realize that they have control issues. My family don't, so it took me a long time to see it for what it is in my husband's family.
The problem is one of his sisters, who acts towards me as if I joined their family as a hanger-on and am still an outsider (after 27 years), while I feel I started a new family with my husband. This sister was so friendly and lovely at first, but she has tried to take control of so many aspects of our home and family life, and has been openly angry with me if I say I do or want something differently to the way she does it. I have always turned the other cheek to avoid a scene, but while I don't fight with her, I don't do things her way, either. She has expelled her own husband and son, and provoked anorexia in a daughter after the girl was chubbier than others at a school dance, and my husband told me how she has encouraged him to leave me (he cut her short and told her not to do it again. He and I have a pretty happy relationship).
We are estranged from her at the moment, after a traumatic incident caused by her entitlement and negligence, but she wants to come back now. She wouldn't speak to us at all or talk about this incident for 6 months after it happened, or make any attempt to see how it hurt us, and now she wants to go on as if nothing happened. I'm sure she just wants her brother back, although she was capable of being openly very rude to him before the estrangement.
Also, now other family members are getting involved on the sister's behalf and telling us to hurry up and fix it because “family is SO important!” It's been made very clear to me that this sister will NOT ever talk about what happened, and certainly never apologize, so I guess we’re expected to just move on without any repair of the relationship. However, we can’t resume the relationship without her acknowledgement and understanding of what happened to us. And even if she does do this, I fear being in any kind of relationship with her after what's gone before, seeing what she’s done to her own family, and considering her entitled, angry victim stance and her hyper-formal, disapproving manner towards me.

So, yes! Siblings are very important! But no more likely to be okay in themselves than any other person. And what do you do about the harmful ones who want your relationship to exactly fit the one in their head?