Having one child is hard enough. Having more will lead to more mistakes and problems.
Sharing is overrated and it's generally better if people have their own property with no strong pressure/expectations to share it. They can share when it's convenient, but when it's a problem then stop sharing. And the borrower can be happy to borrow things sometimes, but realize sometimes other people's stuff won't be available. This is the same as how two adults friends would typically treat each other.
Sharing space is problematic too, not just possessions. It's typical for adults who share spaces to either have conflicts about the spaces. The main things that prevent this are being flexible and conflict-avoiding rather than picky, and having shared culturally-standard expectations about how the space is used (e.g. most adults in our culture have a similar concept of what to do and not do in a kitchen space or bedroom space).
Sharing rooms is typical in our society. Even if everyone has their own room, they still will share a living room, dining room, kitchen, etc. Usually parents mostly get what they want regarding shared rooms and children have to defer. That's somewhat bizarre because parents are the ones who are much better at dealing with problems, finding alternative ways to get stuff they want, etc. Parents are better at delaying an activity until later and dealing with life over a longer time horizon, and have way more other options, so usually they ought to be the ones deferring about the use of shared spaces (unless child is happy to defer in this case). What about two children sharing a space? That's hard and our society causes that difficult situation to happen far more than necessary – then complains that children get upset too much, squabble too much, etc
Children are people and ought to be treated like full people. So they ought to be able to choose their friends, rather than being required to be extra close friends with their siblings. This is often problematic in terms of various resources like money or parental attention to multiple children in different places all wanting help, now, with their separate projects.
Once you have a situation with several children trying to get along with each other (no other choice), sharing stuff, etc, then what has to be done is help them learn skills to deal with this situation. These are difficult skills – most adults have lots of problems with skills like these. It sucks to pressure children to learn these particular skills at a young age or else have ongoing conflicts with their family. It'd be better, in general, if children learned to deal with siblings after they learn how to deal with people more at arms length (which is an easier place to start). Yet getting along with siblings can be learned and children can be resilient to all kinds of difficult situations.
Children can forgive and deal with a lot – people massively underestimate this because other stuff is going wrong (coercive parenting, coercive schooling, treating the child like he's sub-human, etc) which is using up most of the child's coping and problem-solving ability. If parents would act less like irrational, cruel rulers then that'd free up tons of child's creativity, energy, good will, etc, to be used on smaller problems like learning how to deal with siblings.
When dealing with sibling problems it's important to keep in mind the perspective I've outlined. That's not just being negative, it's part of the solution. The key to fighting with siblings less is to lower expectations – recognize it's a tough situation and be less ambitious about what one expects from it. Just like if you live in a poor family, it helps a lot if the children recognize they are poor, recognize that's bad and hard, and calibrate their expectations accordingly. Children in poor families can be happy if they learn skills like sometimes being OK with not getting to buy something. Children with siblings can be happy if they learn skills like sometimes being OK with giving up a shared space or shared possession. Children can learn standard coping strategies for how to do this – e.g. have a list of activities they like which they can switch to which use a minimal amount of space, are flexible about where they can be done, and don't require any shared possessions. For example, if each child has their own phone/iPad/laptop then they can watch movies, watch youtube, listen to audio books, etc – and those activities will always be options they can fall back on to avoid a conflict over a shared space.
There are lots of other things to learn that also help. Like how to communicate one's preferences and make clear statements about what outcomes one is OK or not-OK with.
I think a lot of problems in multiple-children families are because everyone involved thinks having multiple children in a family is normal or good, and they don't see the problem with it. So they aren't putting effort into coping with it. They think it should work better than it does work, and their unrealistic expectations lead to ongoing fights.