In a letter from a reader who is struggling with the challenge of trying to give a "witness" to the real truth (to his JW family), I have below an excerpt from that letter, because I believe the scriptures will help all the other many readers, who write me, and are dealing with the same thing (including myself).

 As to why your family (and my daughter and the rest of "witnesses") are blind...Remember 2Thess.2:9,10,11,12; Rev.12:9; and 2Cor.4:6? The fact is that we are all blind, and no match for Satan. Unless Jehovah graces us with truth, we will not be saved. He is generous in the extreme for those who have a complete heart toward Him (Psalm18:25). But if we don't prove to have such a heart, Satan says, "He's mine". This Jehovah allows, because it is just.

So when you say that the reason why your family can not grasp the real truth, and be released from the false doctrines of men, is

..."I think it's just a mental capacity...and no amount of holy spirit is going to change that"

...the Bible indicates that it is just the opposite...Holy Spirit alone can cause truthful insight...and this it can do no matter what our mental capacity (1Cor.1:27). 

But for those not pure in heart...it doesn't matter if you are a genius...you will never grasp, perceive, or believe. 

We hope for some, the blindness caused by Satan's veil can be removed as it was for us...through the witness that we try to give. Very few will be saved, according to prophecy. 

"All the earth" worship the wild beast, and they are not written in the Lambs scroll of life (Rev.13:8). 

This indicates that any left inside, after the command to "Get out of her, my people" is given (Rev.18:4; Jer.51:6), and the "disgusting thing" is clearly defined and "discerned"(Matt.24:15,16), will be judged adversely, and thrown into the wine-press (Rev.14:18,19,20; Jer.2:21; 25:29,30). Jehovah would not do this to them if they did not deserve it. He would not judge them based on mental capacity, but rather on a heart that takes pleasure in unrighteousness (2Thess.2:11,12).