How does FSI ( fluid structure interaction) work?

In a nutshell, you need to perform a flow simulation for blood using non-newtonian fluid properties which represents the blood. Then, export flow sim results to structural analysis (tools>flow sim>tools>results>export results to simulation (if I remember correctly)). Flow results with be extracted in terms of pressure and shear stresses to the structure and with be your input loads in sim. For the flexible structure, you may need nonlinear module for stress analysis. Hope it helps.

There's not a straight-forward approach to this. Since the structure is flexible, it's response is highly non-linear to the transient pressure waves the fluid will induce.  Therefore your structure's stiffness will vary greatly with the applied pressure.  Re-define the scope of your project and simplify the problem, if possible.

Are you interested in a final deformed shape?  Is there a comparable solution to another existing piece of geometry you can compare to? Or are internal structural stresses the final product you wish to gain? Is the flow assumed to pass through or by the structure such that pressures behind it will influence the stiffness response?  Or is this more like a fixed flow (laminar or turbulent?) against a "plate?"  If you assume flow conditions, simply, as generating a defined pressure shape then you need only craft the final pressure profile as a boundary condition on to the structure itself and forgo CFD.

Or, you may need to use a simplified CFD analysis to generate the fluid pressure on to a "dummy" structure in order to gain insight as to the pressure distribution.

If, however, (returning to the first paragraph) the structure's shape will change significantly then the fluid regime will change and, hence, the fluid pressure will change.  This is considered two-way fluid-structure interaction and is presently not supported by these tools.