When a younger sibling has a shidduch before an older one, the Rebbe's guidance:
The general order is to marry off the older one first, following halachic sources and Chassidic custom, and parents should make every effort for the older daughter (IK 2128).
With the older sibling's sincere consent, one may proceed with the younger one's shidduch. Ask the older sibling's forgiveness, be sensitive to their feelings, and trust in Hashem's providence for both of them (IK 936, 7642, 10576). In one case the Rebbe also advised setting aside money for the older brother's future wedding (IK 3628).
Discussing a match for the younger one is not the concern; the question arises mainly at the stage of the actual wedding (IK 2460). For a specific halachic question about the order of marriage among siblings, consult a rav (IK 7076).
Decisions about an older sister's shidduch should not be made without her consent. Involve the parents and find an energetic shadchan (IK 10922).