First of all, I don’t blame you for your concern. It’s absolutely necessary to carefully inspect your own concern before the big decision.
Let’s just focus on the practical matters instead.
1. I think you already understand that it’s difficult to meet a good person. If you say your GF is really good except for her mom issue, you’ll know that it’s not easy to meet a better person after all. This isn’t just for you. Many people, before marriage, think that s/he is absolutely wonderful only if … If one can go over that …, they marry, otherwise end the relationship (sometimes with a long tail).
2. After all, your concern seems to be “potential financial loss due to the childish/selfish mother-in-law”
At this point, I want to ask you. Would your attitude be different if she is not childish or selfish? I’m not necessarilly saying she is. I don’t want to make that judgement solely based on your description so far, but you certainly think she is. But that’s not the point of this question. If she had been a really supporting mother to your GF – she worked hard to raise and support her, but she was unsuccessful in the end and she ended up in a poor situation like this, and needs her daughter’s help financially. If that was the case, would you accept the potential financial burden and marry her?
If your answer is No to this question. This is no-brainer. End the relationship.
If Yes, let’s dig deeper. (still with the assumption that your mother-in-law was a good person) How much do you think you should spend to support her? $300/mo? $500/mo? $1000/mo? Based on how you describe things and write, I assume you’re well educated and competitive worker – thus making pretty good money after all. You say bay area, so maybe you feel like you’re still poor, but I guess you would be still high in the salary curve overall. Say you give her $500/mo. It’s $6000/yr. Can you spend this extra money for your wife?
You may not welcome it but can you still love your spouse while you have to spend that much money to support her family?
Ok, again, if you say No here. It’s no-brainer again.
If you still say Yes, I have a few comments:
* If you’re an average engineer or above in Bay Area, the money itself wouldn’t be the biggiest problem.
* Based on your description, although your mother-in-law didn’t help her daughter financially, but might provide good enough emotional support to her daughter (otherwise your GF will not act like she does now). She may not as bad as you currently think. Family matter is very complicated and a norm of one family is hardly transferrable to the one of another.
* You don’t need to spend that money for your mother-in-law. You’re spending that money for your GF (or spouse). In the end, what’s the difference if she gets happy for doing that? Some person spends well!! beyond that money on more ridiculous things. At least supporting your irresponsible parent isn’t really one of the worst in my opinion.
* Finally, yes, you don’t have to do this, and it might be true that your GF’s mother is just a bad person. But she didn’t come here and ask for opinions but you did, and I’m giving my opinion.