It sounds like you're worried she's going to change her mind after she is done with her travels and then suddenly want to be in a relationship again. In which case, she shouldn't give up on dating now. But even if she does change her mind later, it would be because she wants to make that choice for herself. Just relax and trust her. She will find her own way of forming her own happiness in life. From the sounds of it, she's been taking good steps towards self-care by seeing a therapist and not going down any destructive paths (i.e. alcoholism). Traveling sounds like it would enrich her lifestyle.

Coming from personal experience, it actually takes a lot of pain and also a lot of determination for someone to openly declare that they don't ever want to be in a relationship again. My older sister was also very shocked and I could tell she wanted to persuade me otherwise. My best advice to you would be: Just be there to support her through this. I'm sure that your sister will really appreciate having your support, regardless of how things pan out in the future. There is no worse feeling than that of disappointing someone you love with a choice you really want to make for yourself.

You sound like a very caring sister. So I really hope that you can see that your sister will be fine and she will be happy. Be there with her so that she doesn't feel abandoned again. She needs all the support she can get from her friends and family. And I say that because it is what I wish for every day.