Fiscal matters concerning a friend or relatives death can be extremely delicate and fragile. Therefore, no matter what you decide to do, it is always wise to approach with caution and consideration.
The fact that you have contributed money to the burial of your friend is a very thoughtful and considerate thing to do, especially considering the financial burden a funeral can place on a person's family.
However, it is not fair to assume you responsible for any future/hidden costs.
Your contribution is enough of a gesture. You should not have to feel guilty or unreasonable for saying no, although this can be difficult due to the situation you are in.
The best thing to do is to sit down with your friend's sister and explain to her that you are not financially in a place where you are able to continue to contribute.
If you explain this to her in a rational way, whilst still being sympathetic and loving towards her situation, there is no reason why she shouldn't understand, or that you should feel guilty.
I hope this helps.