It’s actually a very good strategy to come out to people a little outside your main circle before coming out to the people closest to you. It’s like a dry run, and if they aren’t accepting, it’s not quite as hard as it can be with close friends and family.
It sounds like your mum might have an idea you are somewhere in the lgbtq umbrella, and it sounds like your sisters either won’t care or will be supportive.
Coming out to ANYONE is a big deal, and you should be proud of yourself for that. It’s a process, it takes time for some of us.
When you think you’re ready, figure out one family member you feel is the most likely to be accepting, and go from there.
The only caveat is if you might get kicked out or otherwise cut off financially, if you rely on them. Safety is the #1 priority at all times.