¡Hola a todos!
We've had a few questions come in on this thread, so I'll split my answer up according to the questions; that way, this will hopefully be easier to read!
1. Daniel-rF's initial questions: suitcases, flowers, books, and people
Al22 is right on the money with his explanations, but since you're still struggling, Daniel-rF, I'll try to approach things a bit differently, to see if that helps.
One of the key things to keep in mind about ser and estar is that which one you use gives a different impression: using ser gives the impression of things being set or fixed, while using estar gives the impression of things being changeable.
With that in mind, let's go through each sentence one at a time, starting with the estar ones.
Tus libros están abiertos.
Estar fits here because the state of a book (opened or closed) is something that changes a lot - people open and close books very frequently, so we don't expect them to stay open or closed forever. Thus, estar shows that the current state of the book is open, but this isn't permanent.
Las flores en tu jardín están todas secas.
If the dryness is temporary, estar fits because this characteristic is not permanent, and isn't expected to be permanent: you can water the flowers and they will no longer be dry. If the dryness indicates that the flowers are dead, estar still fits because we're talking about a state of health/well-being. That's because a living being's health and life aren't looked at as being permanent things: they are expected to change (that is, people will get sick, get healthy, and get sick again; also, people will live, but at some point, they will reach the next state and die). So estar reflects that non-permanent nature of health and living.
Los hermanos míos son peores.
Ser fits here because we're talking about a trait of badness that we consider to be identifying or permanent for these brothers. In other words, they are consistently naughty, and we don't expect them to change. We consider their badness (or their being worse than others) to be a constant. Thus, we use ser to describe them. If they brothers were only being naughty temporarily, though, then we could use estar to show that.
Las maletas tuyas son pesadas.
Ser here indicates that we consider these suitcases to be heavy - it's a defining characteristic, and we don't expect that to change. This could be because the suitcases are just heavy suitcases (even when they're empty); however, you will hear some native speakers say this about bags that are packed up for a trip too (i.e. the suitcases are packed full and are to remain so). Using estar in this sentence is also possible, and it would clearly demonstrate that the heaviness is just temporary. If you think about going on a trip with a full suitcase, though, you can see how its heavy nature could be considered a constant (that is, you don't expect it to go from being heavy to being light).
2. Scott_C's questions: marital status
You can use either ser or estar to talk about marital status in Spanish, so this one gives you some flexibility! In general, using ser just presents a person's marital status as a character trait (i.e. as something that defines them), while using estar presents it as a state (i.e. as a stage that a person is in now (but there are multiple stages, and this may change)).
As a result, saying Mis abuelas están viudas for “My grandmothers are widowed” would be unusual, since being widowed is something that tends to stay with and to define people to some degree; it's not generally thought of as a state that people are currently in and will move on from (although widowed people can, of course, get married again). It's more just the nature of this particular marital status, and people's attitudes towards it, that makes ser a much more common choice. (As for why being widowed doesn't fall under the rule that estar governs health, life, and death: since being widowed is a marital status, rather than a reflection of the widowed person's own health or well-being, it doesn't really fit into the health/life/death category (even though their spouse has died).)
Along the same lines, saying Mis padres están divorciados for “My parents are divorced” is more common because it shows that the parents are in a divorced state in terms of their relationship. If you were to say Mis padres son divorciados instead, this wouldn't be wrong; however, it would be looking at being divorced as one of your parents' personal traits, and that's less common.
There is a section about marital status in the Survival Kit lesson Family Matters which may be helpful, if you haven't seen it already.
3. DawnE26's question: celebrations and Carnival
The difference in these two sentences is really just what we're implying, or how we're looking at them.
As you say, ser is normally used for planned events, and estar is normally used for all other instances of showing location. So ¿Dónde es la celebración más espectacular de Carnaval? is talking about a celebration as a planned event, while Parece que el Carnaval enorme y tradicional sólo está en Brasil is talking about Carnival as a general festival/holiday, and not as a planned event.
I know the lines seem to be a bit blurry here! This is one of those instances where how native speakers feel about or regard things will impact the verb they choose. If what they're discussing seems planned by nature (like a celebration, a concert, etc.), then they will use ser. If what they're discussing seems more general and not planned (like a holiday), then they will use estar to talk about its location.
I hope that this is helpful! Ser and estar are certainly tricky concepts to wrap the brain around, but they are also one of those things that you start to get a feel for as you are exposed to more and more Spanish.
If any of this is unclear, or if you still have questions, don't hesitate to ask!
Saludos,
Liss
