I am sorry to hear that your father is ill. Regarding your U visa, do keep in mind that "continued" presence is needed for at least three years if you plan to AOS in the future.
Also, when you attempt to reenter you will be an "arriving alien". If you have any "issues" you may be detained or not allowed to reenter.
So, I would suggest that you discuss your particular situation with an experienced immigration lawyer. Good luck.
It will depend on a number of circumstances surrounding your case. Keep in mind that if you are inadmissible, you can be denied reentry, even if you have a valid visa. You should consult with an immigration attorney before you leave.
www.gassonlaw.com - Disclaimer: This a general answer to your legal question. Unless you have a signed engagement letter with me, you should not consider this information to be legal advice.