It's often difficult to precisely and quickly decribe the relationship between the speaker and a relative (who may or may not be present).
Q: "Who's that in the pale blue shirt ?"
A: "That's Jimmy. He's my youngest sister's husband's eldest son by his first marriage."
"That's Jimmy. He's my sister's eldest stepson."
A: "That's Jimmy. He's my brother-in-law's eldest son ."
All are correct. All convey more or less the same information in different ways. The phraseology can be manipulated to imply the speaker's attitude to the subject.
However, a simple heirarchical code would make it easier do describe exactly, free of prejudice, the relationship betweeen the speaker and the subject.
This is done with a combination of letters and figures in sequence, as the relationship or connection becomes more distant.
M = Mother
F = Father
S = Sister
B = Brother
L = Lad (= Son)
D = Daughter
H = Husband
W = Wife
More codes can be added to this table.
One's birth mother would be M, one's stepmother M1.
One's sisters would be S1 for an only sister, or S1, S2, S3 in order of age.
One's step-brother (older) would be B1-1, but a true brother would be B1. Presuming the speaker is Male, an additional younger brother would be B3.
So to describe "My Stepmother's Husband's second daughter by his first marriage", you would say:
"Her ? That's Jenny, she's my M1D2-1"
Stepmother -> Step-daughter: second: first marriage".
A bit of clarification and rationalisation is definitely needed (sensible suggestions welcome) but the basic concept seems sound.