Can we say a sentence like 'her relation to her mother in law'?
A relationship is a larger concept that includes relations.
"There's plenty wrong with Martha and her mother-in-law's relationship."
"We are in a relationship, I'll have you know."
A relationship can be neither bad nor good when it begins: it can only take on color as it progresses. Relations, on the other hand, can begin as sour (or excellent):
From Man and Superman by G.B. Shaw:
TANNER. You lie, Tavy: you are. So let's have her down from the drawing-room and ask her what she intends us to do. Off with you, Tavy, and fetch her. [Tavy turns to go]. And don't be long for the strained relations between myself and Ramsden will make the interval rather painful [Ramsden compresses his lips, but says nothing—].
Note that you can't say "foreign relationships."
Also: relations (always plural in this case) can mean sexual intercourse.
More: a relation (singular) can mean a relative ("Those morons and twits are all my husband's relations; I have nothing to do with them.")
The word "relation" is slowly falling out of use, for no immediately apparent reason. The word "relationship" keeps marching triumphantly upward thanks mostly to psychobabble being all the rage.
I hope this helps. If someone has a better answer, I'll read it myself with much enthusiasm.