Currently on my third year of H1B visa. If my sibling were to begin a family based F4 GC petition for me now, and the priority date is 12 years away, is there any way for me to continue to stay in the US until the GC is approved after I have used up my 6 years maximum of H1B? Are there any extensions possible based on the pending GC application? Or do I have to go back to India after the 6 years of H1B and wait there until the GC is approved? Also, I should add that the above scenario is assuming that I don't have an employer-sponsored GC petition.
Unfortunately, the law does not permit you to wait the 12 years in USA just because you have a GC pending. No H-1B extensions are available on this basis. You will have to wait outside USA. Of course, you can come back on H-1 once you have reset the time by staying outside for one year. By the way, it is permissible for a person to apply for green cards through multiple categories simultaneously.