I agree with Allzero your profile and credit won't be impacted as long as you are not joint on the application. I wouldn't recommend being a Joint applicant...
Even if OP did open a joint account with said family member, that still wouldn't make the family member a member of NFCU. It's in their policy. Anyway I don't think OP was referencing a joint account, just a "way in" for said family member.
But yes OP, rest assured, you can't sponsor them even if you wanted to unless they're directly related as mentioned above.
I don't think your relationship will be impacted not unless you add them to your account. In this way there will be impact on you. But to be sure I would call and ask customer service then make your decision.
@kkboom I have referred pretty much of my family members and today they are all thankful! bcuz now there credit is well established!