帰省 and 里帰り: a home visit, tickets and a decision before childbirth
Compare returning to a hometown, buying tickets for a later trip, visiting parents on vacation and deciding to stay with them before childbirth. Keep each visit, preparation and decision distinct.
帰省 separates a return from preparation
帰省 (きせい) means a return home or homecoming. In “夏休みに家族で故郷へ帰省した。” (“I returned to my hometown with my family for summer vacation.”), the speaker returned to the hometown with family for summer vacation. 夏休み and 帰省した supply the time and the completed return.
In “年末の帰省に備えて早めに切符を取った。” (“I bought tickets early for the year-end trip home.”), the speaker bought tickets early for the year-end trip home. 切符を取った is the preparation already done; buying the tickets does not establish that the trip itself has happened.
里帰り can describe visiting parents
里帰り (さとがえり) can mean returning to one's parents' home. In “休暇を利用して家族で里帰りした。” (“I used my vacation to visit my parents with my family.”), the speaker used a vacation to visit the parents with family. This is a visit already made.
The family accompanying the speaker belongs to this scene. 里帰り alone does not require the same relatives to accompany every visit or prescribe a particular visiting season.
A decision before childbirth
In “姉は出産を前に里帰りすることにした。” (“My sister decided to stay with our parents before giving birth.”), the sister decided to stay with the parents before giving birth. 出産を前に supplies the timing, and 里帰りすることにした reports the decision to return.
The decision does not establish that she has arrived at the parents' home or given birth. Keep the completed home visit, the ticket purchase and this decision with the particular action each sentence reports.
Does 姉は出産を前に里帰りすることにした mean that the sister has already completed her visit home?
Show the answer and why
No. It reports her decision to stay with the parents before giving birth. This differs from the completed visit in 里帰りした and from buying tickets in preparation for a return home.