I have four parallel lines but unequal Can I make a line in the middle?
with some lacing and levels maybe, not sure, or filter by length
Since you want the hard corner on the landing, start with the stringer lines (highlighted blue in your image) instead of the riser lines.
Use a PolyCurve.ByGroupedCurves node to generate a list of PolyCurves - one for the right and one for the left stringer.
The use a PolyCurve.Points node to pull the points defining each segment of the PolyCurve.
Use a List.Transpose node to shift things so the points are in pairs instead of triples, and then use a Line.ByBestFitThroughPoints node to generate the ‘cross lines’ between each segment of the stringers.
Now pull the midpoint of each crossing line with a Curve.PointAtParameter node using a parameter value of 0.5.
Finally generate a PolyCurve though each of the midpoints with a PolyCurve.ByPoints node.
If you find that your crossing lines are off you’ll need to reverse one of the two triple sets of points.
Give it a shot and see where you get. 
Reverse one of the polycurves before you pull the points with a Curve.Reverse node.
Hi @Alisadeq here is one dirty way, can probably work as well in your case
stairline26.dyn (37.4 KB)
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Thank you
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