Is there any halachic source for not dating/getting married before one’s older siblings
Is there a non-halachic reason for this ?
Obviously one feels for the older sibling , but is that enough to stop a younger sibling from starting to date?
This is a very sensitive and emotional issue. On one hand we don’t want to insult the older sister, besides there might also be an issue of respect for the older sister, but on the other hand the younger sister may have a strong need, and it may be very important for her to get married and not wait around.
The sources for this are partially based on what Lavan told Yackov “We don’t marry off the younger sister before the older one”, and that the daughters of Tzlafchad got married in age order. this doesn’t mean that a younger sister can never get married before an older one. Some of these issues are discussed in the poskim. See sources.
If the younger sister wants to go before her older sister and she gets her permission or she is mochel, then it is surely fine.
This is just the basics, but every situation is different and the needs of both have to be addressed and it should be discussed with a Rov beforehand.
May H-shem send Siyata Dishmaya that this should never be needed.
See Rashbam Baba Basra 120a D:H L;halan, and Rosh Kiddushin 1-54, Shach Y:D 244-13 in the name of the Bach, Likutei Shut Chasam Sofer 40. Shut Avnei Chefetz 25, Shulchan Haezer last ftnt. end of siman 1, Shvus Yackov 1-76, Ezer MIkodesh pg. 35, Divrei Yatziv Y:D 9, Nitei Gavriel Shudduchim chapter 9.