Here's the code:
int dayOfWeek = (int) DateTime.Now.DayOfWeek;
DateTime nextSunday = DateTime.Now.AddDays(7 - dayOfWeek).Date;
Get first the numerical value of the day of the week, in your example: Tuesday = 2
Then subtract it from Sunday, 7 -2 = 5 days to be added to get the next Sunday date. :)