Keep practicing. Part of your voice cracks COULD be that you're still going through puberty, and your voice hasn't fully developed into what it'll be when you're an adult. I'm also an alto, and I used to be really self conscious about my voice because I would always have cracks, and I wouldn't be able to hit the high notes. SOO, what I did was… Well, it's not what I did. I somehow became praise leader, and I was forced to sing the main melody. Do scales daily, and try to reach higher. Don't strain yourself though! Also, one thing my friend tells me is to ALWAYS breathe from your diaphragm– try breathing in for 15 seconds, and then breathing out for 15 seconds. Not the shallow breath, but a deep, full breath. If you do that daily, it should help your breathing, and it'll help your sound. More air = more range.
Good luck!

For your voice to be stronger using the higher notes you need to go there and put more effort into it. I used to love watching the Opera performer Jose Carreras – he would stand on tip toe when reaching for the big notes. You don't back off the high notes, that would be like pulling off the road before a big curve. Here's a cool link that I found with advice

1Sing a note and see how long you can hold it.
2Breathe from your diaphragm. Don't ever breathe in through your nose, it will make it harder to reach high notes. Work on breathing with your body and strengthing your diaphragm.
3Test breathing through your diaphragm. Bend over at the waist and sing. Notice the feeling in your stomach and the sound that you create.
4Warm up for a while before starting to sing. This produces a richer sound as opposed to sounding strained. (i.e. When you blow up a balloon, it is much easier to stretch the balloon before blowing it up. Your vocal chords function in much the same way.)
5Find songs that are in your voice range
6Start practicing scales and slowly increase the pitch each run. Eventually you will have a higher range and a stronger voice.
7Using your thumb, push the larynx from side to side gently, allowing your vocal chords to loosen, thereby putting less stress on them when you begin to sing.
8Find a quiet place inside of you and just let the voice you found out.

9IMPORTANT: at the the high notes you must push down to the belly and from the belly to your knees and in the belly also to whole sides down (you must have a backward feeling when you jump from low to up), the larynx must not move when you go from the lower notes to the upper notes (to avoid of crack)-but subjectively you feel when you go up the larynx you push down (this feeling is caused that you are stretching your muscules in the larynx to not move). Your soft pallate must be up and your jaw must hang down freely without pushing.

Edit: hello, read what I wrote, and look at the links, they are vocal exercises. And know this, most singers, the best singers, just don't jump up and start singing. The voice is an instrument that has to be learned and trained. Yes, you have to exercise and practice to hone your skills at singing.

Fundamentally, your singing voice should be a natural extension of your speaking voice. This is what makes it unique and sets up the foundation for a healthy vocal technique. Don't try to imitate another person's voice. Start from what belongs only to you, and then you can work on stylizing it for whatever musical genre you happen to be singing.

Above all, it's important that you are relaxed when you sing and not tense. This is the reason most of us do our best singing in the shower or the car. We can just let our own voice come through without worrying about who might be listening. That said, here are a few practical things that might help you out.

Breath support is important. Without air, we can't sing. But simply taking huge breaths before starting a vocal line isn't going to help anybody. The idea is to maximize the efficiency of your breath. Try not to let air escape through your nose when you sing. This will allow you to sing longer phrases and help eliminate a "nasal" sound.

As far as "strengthening" your voice goes, you already have a vocal mechanism capable of producing more sound than you might be used to. Try speaking as if you were trying to get the attention of someone on the other end of a big room with lots of people in it, without screaming. Notice how it feels to make that much sound, and pay attention to what your body is doing. Then try to replicate that sensation when you sing. If all goes well, they may have to turn up your sister's mic next time 😉

Best of luck!