You are not obligated, no, it is entirely up to you. Of course there will be consequences of not giving money, just as their will be consequences of giving money. You have to weigh one against the other to determine which will have the outcome you want most. Less cost but angry mother, or happy mother but higher cost. Helping uncle's family but stressing your own finances, vs not helping your uncle's family but not stressing your own finances.

I think there are also some other factors you could consider. Is your uncle's financial situation something that he got himself into and could have avoided or could get himself out of by making better decisions, or is it an unfortunate situation outside of his control that he is struggling with through no fault of his own? Is there a way you can help him become more financially stable in the future so he no longer needs help, or would it be an endless cycle that ultimately wouldn't do either party any good and would just allow the problem to persist indefinitely? Does your uncle need an intervention of sorts, or does he just need enough support to get back on his feet? Are there financial institutions or other forms of official aid that can offer him support and options so that you don't need to step in? Can you possibly help him find a higher income job so he can support himself, instead of just offering money? If your uncle is just bad with money, can he be taught how to better save? Could he benefit from a financial advisor? Etc, etc.