There is no point talking to your mum about the WTS. From what you say, she is very committed to them so will not accept anything you say against it. Being a high control parent (like my own mother) I imagine she loves the fact the religion gives her the right to force everyone to live by her standards and beliefs. It would almost impossible for you to change her as she does not sound like she wants to change. All you will do is create far greater trouble for yourself.
However, it is worth giving your sisters some information. This would have to be done subtly as if they are adamant it is the truth they too will create problems for you. There is some information to help you with ideas of what to discuss at http://jwfacts.com/index_files/5min.htm
I recommend a conversation be filled with questions that go something like this.
Why would you like to get baptised? - Because it is the truth
Do you think that the reason you feel that way could be because you were raised a JW? - not sure
When a person is raised a Mormon they also think they have the truth. Do you think it would be wise for them to study about some other religions before devoting themselves to the Mormon religion? - Yes
Do you know if what the Witnesses used to believe 100 years ago is the same as today? - I think it was
If the conversation is going ok you could then continue: There is some amazing things I have found out about the Witnesses. Possibly discuss the symbols they use to have such as pyramids and the knights templar, as shown at http://jwfacts.com/index_files/davinci.htm Talk about how for 30 years the went around telling people Armageddon was going to come in 1914 and when that was wrong the spent the next ten years saying it would come in 1925. It may be enough to get them to think, or if they get very angry with you it will let you know it is not worth bringing up with them again.
One thing- they will immediately dismiss whatever you say by accusing you a reading apostate information. You can quite honestly tell them that that nothing you are saying is from apostates, everything you have mentioned is straight out of old Watchtower books.