Would it be all right to have my sister, who was maid of honor for my first wedding, be matron of honor for my second wedding? She is still my best friend, and I can't imagine being married without her with me, but don't want it to seem odd or uncomfortable for her.

It would be absolutely fine! All you need to do is ask your sister, making sure she is comfortable with the idea. (And if your maid of honor at your first wedding had been a friend and not a sister, the same idea still applies.) You might tell her that you would feel so fortunate to have her by your side, supporting you and welcoming her new brother-in-law. Your upcoming wedding is not a flashback or replica of your first wedding. It’s a fresh start, and who better to have with you than your best friend, who also happens to be your sister.