Is your brothers family closer (distance) to your parents? TBH if my mum told me that my brother and family were coming over for a bbq on a sunny Sunday afternoon and we didn't have other plans then I'd say that we'd come over and join them, but then we only live 15 mins away.
We don't do much socialising with other people although DP goes out with an old friend for a curry most weeks.
I see my closest friend after school every few weeks with all our kids, at one point it was a couple of times a week but life gets busier. I see another close friend, again with kids, for a meet up on most school holidays. Both only live in the next village so it isn't distance.
Apart from that we have a meal with some of his old friends once or twice a year, he saw a lot more of them when he was with his ex, and have been out with one of my school mum friends and partner once.
We do however see family. Both my parents and his mum live about 15 mins away, we see my parents at least fortnightly and he pops in to see his mum most days (although that's mostly caring). His sister and cousin are often at his mum's (every few months) and we will go over for a meal when they are.
The difference probably is that we are happy with this.
I didn't really socialise much when I was with exH either. TBH work friendships were enough for me and outside of that just having exH and then the kids around was enough. I did find on maternity leave I was more lonely, but that's when I met the two good friends mentioned above.
DP and exW socialised much much more but when I've asked him he says mainly it was because it wasn't good when they were just home together and being out with other people was better.
It's the same with holidays, he and exW always used to go away with other friends and families, we only ever go away on our own or with the kids and the one time we went with SIL & BIL (just adults) I wanted space and time on our own.
If you aren't happy about it you need to do something about it. Friendship and people aren't going to come to you, you have to go out there and grab it.
My mum and dad moved 200 miles to be near me when I had kids, my mum went out and joined things - WI, local pantomime group (she had no interest at all in performing but helped with the costumes), local village magazine, took up bowls and really enjoys it, helped with a new village centre and is now a governer at the village school and on the parish council.
My dad does almost nothing and nearly everyone they know is through my mum.
She never seems to be home!
I was talking to her about it the other day and age said she felt she had to get out there because she is basically lazy and if she didn't then she would just be at home doing nothing and knowing no one, and when you work and have kids your life is full and that's OK, but she was retired in a new place and would be lonely and bored doing nothing all the time.
My DP is dying and won't be with me for all that much longer. While the life we have and the future we planned together was enough for us, it won't be enough for me on my own. I don't need lots of people but I do need a few, close, people. I know that in time I'll have to get out there, much like my mum did when she moved.
If what you have is enough then embrace and enjoy it, if it isn't then do something about it.