if you have two triangles you can prove them congruent by stating that all of the sides are congruent, hence (SSS=Side, Side, Side). You can also do the same by stating SAS (Side, Angle, Side) or ASA...
Why don't you just simply say...on my mother's side or my mom's side. I think the word you're looking for is "enate", it refers to one whose lineage can be traced through the female line, i.e. "MOM".