Got to agree with Stuart 45. Render to reveals and head of window have all been chopped of to get window in. They have then used timber rules and rendered reveals and head. Last thing at night, struck rules and blended edge in. Not made bad job of it either. This is the reason you have not got a lot of meat left on reveal and head mullions.
Now if you look very closely, left hand side of window, you have hairline crack running from top corner of window at approx 20 degrees up to roof verge.
On the right hand side you have hair line crack running from top corner of window at approx 65 degrees for length of 300mm, although I can not be sure of this one. My eyes are old now, one minute I see it and then it fades. The centre crack is also going up further, but difficult to see how far.
These are all the symptoms of deflection due to inadequate support. I am not a structural engineer, but have enough years under me belt to say, in my opinion she is not going to go any further.
If your parents are going to continue to live there for a period of time, then I would just make centre crack good, rub a bit of painters flexible caulk in the hairlines and tosh another coat of paint over whole wall. Fix a tell tale over crack. Find a good class tobacconists and buy an old fashioned clay pipe. You can stiill buy them. These make the finest tell tale possible. Fix the bowl of pope with airadite one side of crack and stem of pipe other side of crack so that middle of stem is over crack. Any movement at all will crack stem of pipe. This is the one of the most sensitive tell tales possible, unless you buy one of these modern, high flaluting, all whistles blowing measuring ones.
It looks like the texture is a heavy coat of tyrolean finish, which will be difficult to patch, but any lad handy with a bit of sand and cement should be able to make half tidy job of it.
If your parents move, and want to sell, Fit two or three good length stitch bars over frame make good, retexture whole of wall and paint.
Hope been of help.
old un.