Has you always felt like this? Were you ever close to your sister? What happens when you contact your sister? If you suggest a meet-up?
TBH, I wouldn't read anything into not being close with young adult nieces and nephews, I think that's fairly normal.
It sounds like you are making an effort with people, which is good - if you don't ask, you don't get. It could be that you've just been unlucky, and chosen people who are unsociable / very busy. So don't give up.
I'm more comfortable with friendships that develop naturally - you meet someone at work or a hobby group and it just develops. It might be that you're looking for something different to the people you're talking to. For example, I have perhaps 3 friends (by that I mean other women I would have a night out or a coffee with, not very often mind). They are all quite busy / not particularly sociable, so we suit each other - no expectations of regular nights out or hour long phone calls for example. I would shy away if I got the impression someone expected much from me in the friendship stakes, as life / work / family stuff are full on. So it doesn't necesarily reflect on you, it can just be where people are in life.
If you meet people through hobbies or interest groups, or volunteering, you are probably more likely to meet someone like you, and a friendhsip can evolve naturally, both feeling your way in and setting the tone and intensity as you go along.