I know how it can feel to be lumped in as the same as others. my parents did the same when I wanted a dog. I can see their stand point now as I am older. I don't know how old you are, but unless you're able to get a job your parents will likely have to forfeit most of the costs. I think this is possibly what your parents (and most) are hesitant on. startup costs are expensive, vet bills are expensive, food, toys, hay, litter, it all racks up. I can understand why your parents say no, they do not have a never ending pot of money. along with that, while you did not want the chickens, you guys already have chickens. they can also become costly also, perhaps rabbits will be too much.
your parents may not even want rabbits, but one (or both) wanted chickens hence why they got them. a parent doesn't want to be stuck with an animal it doesn't want when the novelty possibly wears off. a lot of the time children have pets because their parents also wanted them, usually it isn't based solely on the kid wanting it.
your sister hasn't done many favours neither for getting chickens and then leaving it onto your parents to deal with.
I'm not saying do not get a rabbit ever, but depending on your age it may be too much work, but also too much strain on your financial situation.
somebody else's pet is a lot different than your own. sure, a month is a long time, but once again it goes back home. you likely had been given food, bedding etc, so you wouldn't have to pay for anything. vet bills were probably not a thing neither. somebody elses pet always feels better than having your own, in the end you give it back.
I think these are some general questions to think over..
- who will pay for bedding? food? litter? toys? hay?
- what happens when they are ill? rabbits usually require either a rabbit savvy vet, usually an exotics vet. your average vet isn't knowledged enough on rabbits and could (and often do) give ill advise leading to a rabbits death. I know this is very morbid to talk about, but rabbit savvy vets can be an hour drive away, and they are expensive. a consultation may be £60, not including medication on top... thankfully I have a rabbit savvy vet that charges only £35 per consult, but every time one of them has been sick, I've ended up paying £60-150 roughly. but bills have gone higher, for some people even into the thousands.
- who will pay for vaccinations? prices vary all over the country, but it seems to roughly be between £30-60 usually. rabbits require two vaccinations, so this could be £60+ every year.
- an out of hours vet can be necessary, and always costly. do you have the money for a £200 consult fee and who will be paying for that?
- who will be paying for the housing? rabbit housing needs to be large, its recommended these days (by the RWAF) for 10ft x 6ft housing total. small hutches that you see on the market for cheap are not enough, nor are they sturdy enough. this can cost £300+ to get.
- what happens in the bad weather? are you really happy to be outside, your fingers hurting from the cold, you are soaked from rain, cleaning out a rabbit pen? I can tell you, it was hard for me as a 13 year old. I often didn't want too. along with that they will need hay refills, bottles checked that they're not frozen etc. you will be going outside many times a day. its hard work and sometimes I wish I didn't have a rabbit. when I first thought about this, I was thinking "nah I'll be fine", do not underestimate how hard it can be.
- what about school? homework can become never ending as you go up in the school years.. it is difficult and gruelling. can you juggle having to clean out a rabbit on weekdays and school work?
- social life..this sounds very stupid but you can't go off out to a party, meet up with friends etc, and put off the rabbits. they are dependant on you. it can mean on school days or even weekends you may have to put off doing certain activities to care for the rabbits.
- plans for university? college? what happens then? rabbits can live for many years. it would be unfair to go off to uni and leave your parents to take care of the rabbits. its a big thing to think about and even far away, but its something to keep in mind.
please don't take this do not ever get a rabbit, but they are expensive animals that need a lot of space. they're not simple, and they can become ill in the blink of an eye and require immediate veterinary attention.
unless you are 100% complete able to care for the rabbits, such as holding a job, then I can understand your parents concerns as ultimately they will be the ones paying out of their pocket, and not yours.
if you can find fair solutions to the above questions (and not - my parents will take care of them, my parents will pay for it as that is likely the issue already), then write them out and present them to your parents.
if you're looking for reliable information, the RWAF is very good and a highly trusted source. they also have a map of their approved rabbit savvy vets, you can always rely on them.