ilayja

Fabian ... jai yen yen, ok? mai kit mak! You are not a bother, you both will figure this out together.
For the rest ... you are looking at this thru the wrong set of glasses, she is doing exactly what she is supposed to do, that is how she was brought up. You cannot tell a Thai woman that it is NOT her responsibility to be thankful for the ultimate sacrifice that her parents made to give her life and to take care of them. That is solely a western value, not a Thai value. Nam-jai (generosity) and gat-tan-yu (honor your debts) are two core values in Thai culture that do not have equivalents in western culture. Her fiancee/husband must accept and appreciate how important this is to her, and make sure that their relationship never comes into conflict with this, because he will ultimately loose. That is why she said it might be better for her to stay in Thailand if he cannot do this. It doesn't matter when or how this was brought up, she brought it up at the right time according to her belief. Telling her to abandon her entire belief system by thinking like a westerner is not good advice.

These responses are expected from western-culture attitudes. So now that all the farang have chimed in, I will give you my 2 baht (having a Thai finacee/wife for over 3 years now). From what I read I am assuming you are the youngest or only daughter, so he must understand that YES IT IS your and soon to be his responsibility to support your mother. That is just one of the many "perks" he gets when a Thai woman agrees to let him marry her. Once you are in the US and get married, submit your EAD paperwork as soon as possible and you will get your work permit in just under 3 months. It doesn't take over year.
He needs to understand your culture and responsibilities, and adjust accordingly. If he cannot, then your marriage will not last very long. There should be plenty of samee-farang in California he can talk to about this, there are quite a few of us here on VJ that would be glad to explain all the responsibilites of having a Thai wife, but also in the same breathe how amazingly wonderful his life will be with a Thai woman who him.