First of all I dont think you are forced to tell anyone you're bisexual.I am bisexual too and I dont think it is so important to tell people.It is your life a the end of the day.Imagine if people told their parents they are heterosexual?No need to.But if you really want to tell them cause you dont want to keep it for yourself just tell them you have something important to say.And then just tell them you are bisexual.If they hate on you or say religion doesnt agree and stuff like that just tell them that it is your life and they cant tell you who to like.If they cant accept you like you are then I dont think you should even try to make them accept you.And most importantly dont let them tell you that you are not normal and that you should change! Ever!
You should ask them to sit down with you at some point of the day, as you really need to talk to them. Make sure it's a day when they both seem in a good mood, that might help. Then just tell them how you really feel, there's no other way to solve this, really. Good luck!
If it's important to you that they would know, then you should tell them. However, before you do so keep a few things in mind and think about it thoroughly first. You're still young and at your age you're still discovering certain things. I'm not going to say you wouldn't be bisexual, because you are what you feel you are. If you feel like you are bisexual, then you are. I'm not saying you're too young to know your sexuality, but I'm curious what makes you think that you're bisexual. At your age it is normal to be curious and to wonder about both genders. This does not automatically indicate that you would in fact be bisexual. I think there's a possibility that you're bi-curious. As a teenager it is totally normal to go through that stage. For some this curiosity is more like a one time thing, while for others they find out that they are indeed not straight. If you'd like to talk more about this, you can always message me and I can try to help you to figure this out.