Is there a ruling that prohibits a brother from marrying before his sister, and is the sister sinful for delaying her brother's marriage due to her special circumstances that are delaying her own marriage?

There is no legal impediment (shari) for a young man to marry if his sister's marriage is delayed, or vice versa, as there is no connection between the two matters. His marriage might even become obligatory if he can afford its expenses and fears falling into temptation (fitnah). The sister does not incur sin if her brother delays his marriage, unless initiating marriage was obligatory upon him and the delay was at her request. If the impediment is financial, it is advisable to work towards facilitating the expenses and completing the marriage.