You can get cavities under a filling it does happen. It would not be any one fault. I am not sure about sealant. I would think that if you had a cavity before the sealant was put on they should have saw that with the x-rays like your cousin did. Keep in mind you have the best available source, your cousin the dentist. (if only I had a dentist in the family) What did he say about this to you?
Putting blame a side. You still have two cavities that you will need to address. Your fears of going to the dentist is understandable, but will not remove the fact that you will need to still go to one to fix your cavities. Hopefully now that you are able to see not only a dentist but a family member you will be able to be more at lease. Having a filling is painless. Most dentist take care not to hurt their patients or they would not be in business for long. Your cousin is telling you the truth, as a cousin and also as a dentist.
Have the filling done and good luck to you