The best advice that I could give you would to pray about your specific situation. I know that no two situations are exactly the same even if there are similar circumstances. There are many practical things that may need to be addressed such as: Can the family of the two siblings afford two weddings in one year? How will one getting married first affect the relationship between the two siblings? Will the older sibling feel resentful if the younger one gets engaged/married first? - especially if the older one has been in a relationship for a longer period of time. Have both couples gone to pre-marriage counseling to prepare for many wonderful years together?
I don't see that it is necessarily disrespectful, especially if everyone is very open and honest with each other. Don't try to hide things or bottle things up. Just be truthful with yourself and with others and it will make it much easier to come to an agreement that works out for everyone involved.