I’m not entirely sure what the culture is like around there but if you’re afraid of being disowned or bullied, if you’re in an unsafe position, I think it’s best not to. Especially if you’re under 18 with no financial support away from your mother.
I know how hard it is to keep it a secret. But it is sometimes necessary. If it’s taking too much of a toll on you, perhaps tell some people you KNOW you can rely on and feel safe with, and vent to them how you’re feeling. Having a good support group really helps.
However, you can try and gage her views. Watch a television program which has a bi character perhaps, and ask her what she thinks about bi people. Or perhaps just ask how she feels about bi people, pretend a friend of yours came out to you. If she reacts with bad stereotypes, perhaps try and clear it up with a “Well, my friend is bi and they aren’t like that at all!”. If her reactions are positive, well, then you’ll know that you’re probably okay!