"I have two older sisters."
"I have two sisters." 姉が二人います｡ (ane ga futari imasu) The "I" is implied in the sentence when you say あね[姉] (the informal way to say big sister, so it means you're talking about your own sibling or someone you're close to). が is the particle used to indicate sentence subject. The ふたり[二人] is the counter to say "2 people", and "have" is います. ^^
So uh, I have two questions stemming from this explanation. Firstly, it's tempting for me to say that お姉さん is simply a more formal version of 姉, but knowing how this stuff works, I'm guessing context and the particular relationship you have with someone is important here? (Like, if I had to take a stab in the dark, I'd guess that お姉さん is an honorific you might say to someone who isn't literally your older sister, but is someone you respect as an elder woman that's reasonably close to you in age, or something you say to your actual sister in a more polite setting.)
Second question is more related to counting, always a fun one with this language: the つ appendage with the Kun reading, is that for counting inanimate objects, where if you're dealing with people, you swap out the つ for 人[り/-ri]?
With regards to the counters, you've got it basically right, but it's a bit more complicated. 人 does take the place of つ with people, but it is not usually pronounced り, 一人 (hitori) and 二人 (futari) are irregular, while with the other numbers 人 is pronounced にん/nin. Also, there are many different counters that can be used for different kinds of inanimate objects that you will learn later.
Reply to your first question: you're a little off. the difference between "oneesan" and "ane" is not so much a matter of formality, but whether you're being extrapolite or humble. "oneesan" is an honorific and extrapolite, "ane" on the other hand is very humble.
you would use "oneesan" when talking to someone else about THEIR older sister, so as to be polite to them. you would use "ane", when you talk to them about YOUR OWN older sister, because otherwise you would sound as if you're bragging about your oh so very honorable family.
when talking to that older sister directly, you'd usually use "oneesan" - as japanese culture has high respect for all your elders, so using to be extrapolite even in family settings is common.
its the same for parents and younger siblings, and similar rules for appropiate language apply e.g. in many social situations. if this topic interests you, check out "uchi/soto" (inner and outer social circle) and "keigo" (polite language) on wikipedia or the like.
私 (watashi) means "I", mostly used by women and it can also be used by men if they really want to be very polite. 僕 (boku) also means "I", but solely used by men, also polite but less than watashi. 姉 (ane) means a big sister. There is also 俺 (ore), it also means "I", used by men in the casual way. In the sentence「姉が二人です。」 (Ane ga futari desu.) "I" is already implied, so 「私は」 (watashi wa) left out.
わ (o) and が (ga) are just particles. I think you meant は, this particle sounds as "wa" and is a topic marker.
「私は姉が二人です。」 (Watashi wa ane ga futari desu.) is also correct.
more about ～人 counter here:
Exactly that - います is for animate (people and animals) and あります is for inanimate (objects and things). Be careful of calling the categories living and non living though, because plants take あります. It's easier to think of it as "can it move itself?"
Examples from the internet: つくえがあります。 tsukue ga arimasu Meaning: There is a desk.
きがあります。 ki ga arimasu Meaning: There is a tree.
おとこのこがいます。 otoko noko ga imasu Meaning: There is a boy.
ねこがいます。 neko ga imasu Meaning: There is a cat
I got the sentence right but now that I'm reading your comment I have a question. If つくえがあります is "There is a desk", then why doesn't our sentence read "There are two sisters" instead of ”I have two sisters”? Where is the part that indicates possession/have and not just exists?
あれ が 二人 います
つくえ が あります
Is my question clear? I know this was a year ago but I'm hoping you're, or somebody, is still here! Thank you in advance.
For animals in specific you have ～匹【ひき】 and ～頭【とう】depending on his size。～人 is used for people or anything you can describe as humanoid.