@Hannah24,
As someone who is in their 20s and queer, let me give you a bit of advice.
1. Realise that sexuality is an incredibly fluid thing that doesn't just orientate between how "gay" someone is and how "straight" someone is. Very rarely bisexual people are 50/50, there is usually some preference somewhere. Many people find that their sexualities move and change - or that they feel that they've discovered terms that fit their sexuality better than before. For example I know people that originally identified as bisexual but then decided they were pansexual (i.e. interested in all genders, so those that don't fit on the male/female spectrum as well as men and women). I myself identified as bisexual, before I thought I was fully homosexual, before doing a little research and finding that
asexual actually best fits my identity. Often members of the LGBT+ community will just use the term "Queer" rather than saying one particular identity so that they can identify themselves as not-straight, but in the meantime not fixing themselves to one set sexuality. Nevertheless, if you feel happy and comfortable with the term "bisexual" to describe yourself, then go for it. Just know that there's nothing wrong with thinking that it might not best describe you in the future, and that you're completely valid for believing that it best describes you now. After all nobody knows how you feel except for you.
2. Remember that nobody is entitled to know about your sexuality. Society, the media, and all sorts tells queer people that if we don't announce our sexuality that we are somehow hiding, or pretending otherwise. You do not have to "come out" at all if you do not what to. Queer people have a hell of a lot of pressure put on us to come out, but your sexuality is nobody's business but your own - not even anyone who you date in the future. If you do not feel ready to tell anyone, then don't. There is nothing wrong at all to do what you need to to feel safe. If you think you would feel a lot better having told people, then that's a decision for you to make. You can come out now, tomorrow, in five years or never, it would not make you any less valid.
3. Absolutely get involved with your local queer community. I know at first it may seem tricky to find (especially in school), but with a little research and when you find out about certain meetings or groups or venues then it can open up a whole new world. Before I "came out", I got involved in an LGBT+ group at school but I felt it was very much a 'support group' for people who felt bad about it. Fast forward to university and I found that surrounding myself with queer friends made me feel happy, comfortable, and positive about who I am. Your cousin definitely seems like the right place to turn to, if she is openly bisexual then I'm certain that she will have local resources. Of course, there's no pressure to come out if you don't feel ready.
Best of luck, and feel free to ask any questions if you're not sure.