PARSHA & DAF HASHAVUA
וימת נדב ואביהוא (במדבר ג:ד)
בנות צלפחד מנאן הכתוב דרך גדולתן (ב"ב קכ ע"א)
The רשב"ם explains that when the Torah is referring to their marriage then it lists them in the order of their age because the norm is for the older one to marry before the younger one (and that's how it actually happened).
Some Poskim say that this rule of not marrying off a younger sibling before an older one applies only to females.
Males who have the Mitzvah of פרו ורבו there is no reason of holding back the Mitzvah because of an older brother who is not married yet.( אג"מ-מהרש"ם)
Some want to prove this from נדב ואביהוא. Chazal tell us one of the reasons they were both punished was for staying
single and not getting married. If אביהוא would have had to wait for his older brother נדב to get married then why
was he punished.
This is not really a proof because if the older brother refuses to get married then there is nothing to wait for and there is
an obligation for the younger brother to get married. Once נדב refused to get married then אביהוא was obligated
to get married and yet he didn't, therefore, he too was punished.
The ש"ך (Y.D. 244:13) quotes the ב"ח " Both, brothers or sisters should not marry the younger one before
the older one".
R.Moishe Feinstein zt"l (Igros M. E.H.2:1) says that the ב"ח is only applicable if both are engaged only then must
we marry off the older before the younger.
A younger sibling (even sister) may get engaged before the older sibling and get married if the older is not engaged yet.
The (מהרש"ם (ח"ג קל"ו says that this Halacha is not a דין גמור but a Minhag Derech Eretz. Therefore in certain
cases ("בעושה משום דחק") it is permitted.