Your study gradually conditions you to do and accept things that you would never consider in the beginning. They say "you will know what to do" because they know that if they told you what is acrually expected at this early stage you would think they were crazy people. What they mean is when you have been conditioned enough you will judge your families choices and want to distance yourself from them. You will not want to support them because you will think that what they do goes against God. you will feel uncomfortable around them because their choices are different from your choices so you will distance yourself from them and spend less and less time with them. You will constantly feel the need to make a stand for your beliefs because keeping quiet will be seen as a compromise. JWs do not compromise. You be absent from all the family celebrations they hold dear, you will not celebrate their birthdays, be part of family Christmas or attend a military funeral. You will be pulled away from your family.
I am sad for your family as you sound like a supportive and loving daughter who has respect for the things your family holds dear. This religion will ask you to make many sacrifices in it's name and give up a lot of the things that you take forgranted. You will change from the person you are now and you won't even realise it is happening. You will be told that people who disagree with this religion may be well motivated but they are influenced by Satan and you shouldn't listen to them. You will trust the organisation more and your family less. A religion that will demand as much from you as this one needs to deserve the level of commitment they will require. You need to be very sure of that before you drive a wedge between yourself and your family.
These people mean well but they are already conditioned to believe that theirs is the only true religion and there is nothing else. They will not read anything that is critical of their faith and never check the cited references in their literature. Check everything and if they lie to you remember that this religion calls itself the "truth".