I find so much myself in your story. Always trying to be nice to people and all I got from this is that there is more and more stories how people took advantage of me.
I'm also in process of improving on this part and I think I'm doing better and best advice you can get imo is:
Honestly say to people what you think and what you want!
Do it in polite way, be short and explain simple, if they don't understand, then you shouldn't be hanging with that person and avoid them in future.
Like this brother-in-law, call him and say that you need headphones, that they broke or one friend offered you money for it something.
And for sister, just tell her that you have strict working schedule and if she respect you (she don't have respect your job) that she won't ask for that. Like you can do it today and that's it, you have big goals, dreams and really want to make it.
What I realized from looking other examples, like you mentioned your brother, it's not really about what kind of problem we are dealing with, it's just how you set up yourself into others eyes. And you can't do that without telling exactly how things are, what you want and what you think! Problem is that people can't know what you think and that everybody thinks same way as you do (just today I've watched one RIO video where coach mentioned how this concept can be applied in poker
).
So, 1st thing, don't use mobile phone while grinding. Put it on silent and after session when you return calls , say it loud that you were "working" (I was using "playing poker" before, but term "playing" seems not serious) and if you can help them now.
Next time someone wants a favour you don't want to do, just don't do it. Just say that you don't have time for this or whatever. Make some reason or if you don't want it just say loudly. Important is to be loud sometimes.
One solid stuff I'm using is google calendar for productivity and if someone need me something, I just say let me check, look at mobile and say, sorry I can't that time, that're my working hours, I'm very strict following this (it helped me improve my earnings...), maybe some other day. Some people would smile, like, you're using calendar like you're doing something important and not playing cards, but when they see you're serious they will start to value yourself.
Best way is to pay attention to others who have more success in this field. Like in poker, watch them and apply, even try to test them with asking for some ****ty favour. I've learned the most that way, coz I seen examples where if I declined doing same thing, I would think I would look as an *******, but when you see it's not that way, you won't have problems with that anymore.
"If you don't value your time neither will others"
Same apply for your job, way you look on yourself, your stuff...
Gl and gratz on great day