If you detach from wanting anything from this visit other than to see him alive one more time, it will be easier to deal with. Allow yourself to detach emotionally from the negativity that might be said to you. Because the guilt comments are not about you, they are about an issue inside of the person speaking them. It is not you, it is them. Don't take it personally. Some people are just mean spirited or critical and it is about what is lacking or broken inside of themselves.
And remember, you don't have to explain anything to either one of them, you are an adult. Remember not to justify, argue, defend or explain (JADE). This is you power, so hang on to your inner power. If SM gives you guilt, don't respond, get silent, or respond to her emotion "You wish I was here for another day...." don't let her make you defensive. They are lucky to have you come to visit. So ENJOY it as much as you can and let us know how it goes.