Just because people ask a question, doesn't mean there is a substantive answer to be found...
though it can be created by someone inventive, responsive to readers, and with tremendous knowledge of the subject...
They may be struggling with the idea of their father dating, especially so soon.
I love him dearly and would love to spend the rest of my life with him, but not sure if I can take not being the true love in his life. His response to something like that might tell you whether he really is emotionally ready to make another lasting commitment.
Similarly, he should not hide you from his in-laws, or from anyone else in his life, if he is really serious about having a future with you, and you should address that with him.
Perhaps some of them were even a little nuts before they were widowed (we are changed by our losses... I also think that widowers with children still at home (most of the widowers I know fall in this category) are a bit more justified in hanging on to "stuff" from their past lives and sharing family (like in-laws) and memories a bit more actively.
This is a giant set of exceptions that negates, for me, a lot of Abel's advice.