This would be my "pseudoflow"... very much a brute force method
1. Use Add Days component (under Date Handling in tree) to add 14 days to current date. This new result would be a "newdate"
2. Use Day of Week Rule (under Date Handling) to see if "newdate" is a Sunday
3. If Sunday, SUCCESS... "newdate" is a Sunday. if not, keep subtracting a day using Subtract Days component (under Date Handling) from "newdate", replacing "newdate with "newdate-1" each time, until "newdate" comes to a Sunday.
Heres the flow below:
If you need any more help, feel free to ask.