Kikar has an interesting psak from Rav Shalom Cohen regarding shidduchim.

An Avreich went to Rav Cohen to ask if it is ok for a younger daughter to begin the "parsha" of shidduchim even though she has an older sister that is not yet married and is looking for her mate.

This may not sound like a biog deal to many, but for many in the Haredi community this is a big no-no. Younger sisters do not start dating before all older sisters are married. The only exceptions are in extreme situations, such as they have been waiting too long and the younger is also getting older to the point where it could hurt her chances if she waits any longer, or other similar extreme situations.

In this situation of this specific avreich, the older daughter s 24 years old and has yet to find her mate. The younger sister is 22 and no longer wants to wait. If she starts to listen to shidduch suggestions and starts dating, she is basically being granted permission to pass her sister and get married first, should a shidduch work out for her before it does for the older sister.

Rav Cohen responded that she should definitely start dating and not wait any longer. Chas veshalom, Rav Cohen said, that she wait too long and find herself at an older age and have a more difficult time dating. She should start dating now, but she should get permission form her sister and explain her situation and concerns pleasantly to the older sister.

Rav Cohen concluded with a blessing that in the merit of encouraging her younger sister to date and supporting her rather than holding her back, she too will find her mate and marry soon.

What is left unclear is what happens if the older sister says no? Does Rav Cohen still think she should start dating despite the opposition of the older sister? Whil eit sounds strange that an older sister would say no, it does happen and is not uncommon. In such circumstances, among people who are strict about this, sometimes the older sister is pedantic about the younger sister not dating. It, understandably, frightens her, and makes her think she is getting too old and her chances of finding a mate are decreasing rapidly. The younger sister possibly marrying first can make her look like a problem and hurt her even more.

From his response it sounds like getting permission from the older sister is just a pleasantry and not absolutely necessary, but that is just how I read his words. I wonder what he would actually say in such a case where the older sister refuses..