Well, let's look at that guilty feeling. You say it stems from not helping your grandmother, and that it's difficult for you to help her because your schoolwork keeps you extraordinarily busy. So, what feeling do you associate with this guilt? Is it pity for your grandmother, or is it something else?
How do you feel about your aunts and uncles who are also not helping your grandmother? Do you feel anything towards them because of it?
The Buddha did teach that we should have compassion for everyone, but that also means having compassion for ourselves. He also taught that we need to develop wisdom so that we do not become swallowed up by our compassion, and ruined by it.
You are in school because it will get you a good education, and an educated person is an asset to society and to a household. It will also help to improve what jobs are available to you in the future, which means that you will have the income to better help others. So being in school is a very good thing for you and for others.
If you were to drop everything to tend to your grandmother's needs, you would be sacrificing your education. So something between the two may warrant a look. How many times a week do you have spare time in which you may even just be able to visit your grandmother? Sometimes just a visit can make someone's day.
I'm not very familiar with this situation, so my advice probably isn't very helpful, but I don't want to leave this thread unanswered. But, as Shokai mentioned, there is no concept in Buddhism of "sin".
There are certain actions of body, speech and mind which are considered to be harmful to you, which may lead to unfortunate rebirths. In Tibetan Buddhism these are often called the Ten Destructive Actions. I'm not sure if these things exist in Therevada Buddhism, though. It may be something you'd like to look into. If you're curious, these are the actions that are mentioned:
Quote:
1. Killing: taking the life of any being, including animals.
2. Stealing: taking what has not been given to you. This includes not paying fees or taxes that you owe, using supplies at your workplace for your own personal use without permission, and not returning things you have borrowed.
3. Unwise sexual behavior: adultery and carelessly using sexuality that harms others physically or emotionally.
4. Lying: deliberately deceiving others. How about white lies, or exaggerating the truth?
5. Divisive speech: causing others to be disharmonious or preventing them from reconciling. Do we use our speech to create harmony and bring people together to help each other?
6. Harsh words: insulting, abusing, ridiculing, teasing, or deliberately hurting another’s feelings. How often do we use speech to encourage people and help them generate positive qualities?
7. Idle talk: talking about unimportant topics for no particular purpose. How careful are we about what we speak about?
8. Coveting: desiring possessions that belong to others and planning how to obtain them. How much time do we spend planning to shop and get things? Wanting stuff? Craving recognition?
9. Maliciousness: planning to hurt others or to take revenge on them. Do we plan how to get even with someone that has hurt us? To let them know who is in charge?
10. Wrong views: Strongly holding to cynical views that deny the existence of important things, such as the possibility to develop all of our positive potential, the cause and effect relationship between actions and results, and spiritual truths.