I'm also coming from the other side (a widow).
I knew before DH died (cancer, was ill for a year) that at some point in the future I would want another relationship.
It isn't something we discussed, and nor would I want to have done so - I'm perfectly capable of making my own decisions about my life without his blessing (or otherwise!).
I'm fairly introverted, I dislike spending time in large groups and I enjoy a few things but don't have (or want) lots of hobbies etc. I actively enjoy spending time with the people I love, particularly with a romantic/sexual partner. I enjoy having sex with that person although I have no desire to have sex with a random hook up.
I don't need a partner, I am perfectly capable of living my life without one, but I find that having one enhances my life - perhaps I've been lucky that the relationships I have been in have in the main made me feel this way.
I'm aware of the reasons why I want to be in a relationship and find it laughable that some people think this makes me weak and pathetic.
I was "window shopping" on OLD very quickly, but it literally was just that - I had no profile information and had no intention of responding to any messages.
At about 10/12 months after DH died I was at the stage that I wanted to start reconnecting with the world, we got a puppy, I joined a couple of clubs etc. I also completed proper profiles on various OLD sites and started messaging people.
I was in no way "over" DH, I'm now 3 years on and don't think I'll ever be over him, I adored him, if I actively think about him I miss him deeply rather than just the constant missing which is always there.
I am however in a much better place than I was 1 week, 1 month, 6 months, 1 year, 18 months after his death.
People will go through this process at different speeds, some will never want another partner, some will look for one much quicker, others will find one without particularly looking.
This man may, or may not, be at a stage to move forward with someone new. I wasn't, despite looking for something, and I was very lucky that my new bloke supported me through the myriad of emotions that starting a new relationship brought with it.
I certainly wouldn't tell your sister it is too early for him.