To address a relationship towards something or somebody, in English you can use the word "self", e.g. myself, yourself, itself, etc.
The Proto-Germanic root of the word "self" is *sel-bho-, which originated from the Proto-Indo-European (PIE) root *s(w)e. The same word can be found in Sanskrit, the ancient language of India, ("svah"), in Avestan, the oldest recorded Iranian language, ("hva"), in Old Persian ("huva" or "khva"; e.g. in "khva-data"="lord", literally "created from oneself"). The starting "S" mostly disappeared in Iranian languages. In Modern Kurdish and Farsi, you can still find this word, it changed from Old Iranian "khva" to "xo" and "kho", respectively.
Just a reminder, the pronunciation of the Kurdish letter "x" is equivalent to "kh" used in Farsi and "ch" used in German (e.g. German: "Woche"="week"), e.g. "Kazakhstan" (the word "Khaki" is also mispronounced in English; it means "greyish" or "dusty").
English: I go myself.
Sorani Kurdish: (Min) xom darom.
Farsi: (Man) khodam miram.
So back to the Proto-Indo-European (PIE) root *s(w)e.
After marriage, the family of the wife and husband become part of one family. To address the new relationships within this larger family new words were created. All these new words have their roots in the Proto-Indo-European (PIE) word *s(w)e.
The mother-in-law is the "Self"-Mother:
Old High German: swigar
Old High German: swehur
The starting "S" mostly disappeared in Iranian languages (see Sanskrit versus Kurdish words above), but not always:
The brother-in-law and husband:
Old High German: swâgur (brother-in-law)
German: Schwager (brother-in-law)
Sanskrit: śvaśurya (brother-in-law)
Farsi: šouhar (husband)
Kurdish: şû kirdin ("to marry")*
*In Kurdish, there are two versions for "to marry":
Men say: "jin henan" ("to get/bring a woman")Women say: "şû kirdin" ("to make a husband")