Never say the following:
“Don’t worry, dear. Yout turn will come, too.”
“Someone even BETTER is in store for you!”
“Maybe you should try a dating service.”
“This must be hard for your parents.” (Yes, I have heard all the above!)
I can beat that. I know someone who was told how terrible her situation was (a view which the person did not share). I think its near the top of all-time stupidest comments.
About the topic: I think as with most things, there can’t be a blanket statement. A lot depends on the ages/gender and relationship between the siblings. JFTR, this is just an observation on my part, but generally when siblings date at the same time, I find that they get married in order anyway. Just an observation.