Regardless of you not wanting to drag the children into this

Regardless of you not wanting to drag the children into this, it is your job, not only as a father, but also as a US Citizen, to do whatever is necessary to protect these children. And that means you fight long and hard for custody of the children. Social Services may not be on your side, but the fact is they have confirmed these allegations and a judge should have a completely different view from them.

Also, it is likely that you will not need to involve all the children. Testimony by the 14-year-old should be sufficient, if it is needed at all. Documentation for Social Services may be all you need, though I would recommend the children be interviewed by an independent psychologists.

You need to immediately request an injunction preventing her from removing the children from the state, or even the area, pending a complete resolution of this matter. You need to play hard nose, with no punches pulled. Regardless of what the step-father or the sister was doing, the mother knew about it and was not doing anything. Likely it has changed now, but the fact it did happen and may happen again.

You should also involve all your friends and relatives in this. Every time there is a court hearing, you want to pack the courtroom with as many people as possible to act as court watchers to make sure the judge does the right thing. They will not be able to be in the room whenever the children testify, but otherwise they should be there. You could even ask a high school Civics teacher if any of his students would like to volunteer to do it for extra credit.

So, get out and fight tooth and nail for your children.