You asked for 2x4s and you're getting a few. I also don't think it's what you were anticipating - I got the sense you thought we'd be saying "don't do it!"?. I'm seeing a lot of folks here say you need to figure out what YOU want. I'm on board with that message.
I do want to give my perspective though, as someone who was in a LTR with an age gap. You know my XW and I had 11 years between us. We got together when I was about 23, she was 34. Lasted until I was about 33, she was 44.
You know my story. You know I loved my XW fully and unapologetically and that I wish I had the rest of our lives together instead of the 10 we did have. But even after she left and my life was turned upside down I still do not for one SECOND regret any of our R. Not at all.
I made the decision at about 24 years old that I wasn't going to have children. This was because XW was clear and upfront with me that it wasn't for her, and she told me that if that mattered to me we needed to address it. I decided I would rather be with her than the off-chance I might want kids in some vague future.
So here I am, 35 years old without children and currently single. Do I want kids? Yes. I've always wanted kids. But I also am very comfortable with the fact that I won't be having them and I am okay with that. I made the decision for myself and even though part of me wants them I don't feel I *need* to have them to be happy. That would have been one way to live my life, and I chose another. There isn't one single path in which we are happy. I honestly think I would have been happy for the rest of my life if I was with XW. I also think I will be happy in this path.
I say this all because I'm the younger LBS, so in that sense I can relate to what you are projecting on to Jack.
BUT.
D.V.
(Here's the 2x4 part)
You are projecting. You are assigning him his wants vs needs, and deciding what his future should or shouldn't have. You are taking away his autonomy by pretending you know better than he does what his future should have. It's invalidating. He is a grown-*** man who can make his own thoughtful decisions, and you are not responsible for those outcomes.
Stop deciding for him, and start asking only what you want, and then be thoughtful, clear, and respectful in whatever that want is for you. Speak only for yourself.
And if you are undecided that's okay too. But be honest about it, and recognize that your own wants and needs might change. Stay authentic with yourself throughout and you'll be okay.
And regarding his personality with your family -
My XW was not always a "people person". She could be cranky, blunt, or just clear that she didn't want to be somewhere. I learned after a bit of time together that this improved greatly when I learned that it wasn't my deal. It was not my relationship to manage. Was she cranky with my family? Great - that's on all of them to work out. They can speak up if they feel a miscommunication or even some rudeness. The few times I tried to manage each side it got worse because I would inevitably mis-represent someone's intentions. Seriously, stay out of it. If your sis is ragging on him it's for Jack to speak up to her directly. If he's stand-offish it's for them to approach him. You are not his mother, and you are not your family's caretaker.
Do not manage other people's relationships. When I learned that my life became so much smoother. If a loved one comes to you with a complaint about your significant other the best response in my experience is a "Oh! Yes, you should say something to him directly that it affects you.". Let them have their own relationship separate of you and they'll respect each other more.