haifagirl: I agree.
Purple, what I mean to say is, you don’t know how Hashem is answering your tefilos. Your sister will get married when it is her time, and you will get married in your time. Maybe Hashem is saving your tefilos for the right time! Of course you can ask for whatever you want, but Hashem may say “No” if it wouldn’t be good for you.
Maybe your sister is finishing a degree now and will be more relaxed if she meets the right one in the summer. Maybe her zivug needs to grow a little more to be worthy of her. Your tefilos will help it be good, but you can’t take her not getting engaged yet as a sign that they weren’t answered. Hashem loves every one of your tefilos, and they will all ultimately be answered for the best outcome that will enable everyone to serve Hashem in the best way for them.