I am so sorry to hear of y'alls loss!!

The fact that you are concerned tells me in fact the opposite, that you are FAR from being a bad daughter, and also look at all that you do, while sacrificing even your own needs. However, something needs to be done about that but your are kind and considerate, but need to reevaluate this so you don't snap.

I don't know if she is housebound but she also has to want to help herself. By not wanting to see a doc, MH or regular GP, how is that helping anyone. She needs to at least see the GP, who can also help if depression is an issue. If she is housebound, are there friends that can come visit or activities in the area to take her to to get her out and engaged so the focus is off of y'all. If she is not housebound, the same also goes, trying to get her involved in something. If she won't, there is only so much you can do as she has to want to help herself. If she won't do something for herself, you CANNOT do it all for her and guilt is not a factor; she may or may not move forward in her own time due to her grief. However, at the same time she may be replacing you and your siblings for companionship for her husband that she lost, which is not healthy for either of you, as he cannot be replaced. It is hard to life alone again and it is good for family to rally with a loss, but to a certain degree. Getting her involved will help.

You are doing what you can and even sacrificing that you are feeling impinged on at this point. This is the point where trying to find the right balance is important in caring for another, of what you can and cannot do for your sanity, and for self-preservation. Take a step back to look at your needs, hers, and boundaries that need to be set so the relationship does not go beyond a breaking point, and it sounds like it will if it keeps up the way it is going. How may days can you realistically go out, what other things can she realistically do for herself, or can be set up to help her with, and go from there. Then talk to her about it and stay firm. How good is this for you or her if it continues as it is.