Can you go back to the begining ? Have you got the marriage certificates of both couples ? If not , then you may need to find them to see who their parents are . Are you with me so far?
You cannot run until you can walk !! If you can you can try
which is free, and you can use the advanced search and one or the other or both if you are lucky will come up and look you in the face.
If you have not got the certificates, I would urge you to get them. Then you can go back another generation which will help you. You also need to search for brothers and sisters on the same generation. Gent is a fairly common name in these parts, and some are Jewish so bear that in mind.
If you need any more help , please ask