According to your description, your sister-in-law is more selfish and self-interested. However, if you are still young and don't have a better job, you can try her to get some more work experience. You also said she's nice to you, so she shouldn't be too mean to you.

If you're in your early twenties, you could try working for her. You try to do your job well, and if she pays you a very low wage and gives you far less in return than you put in, don't work for her (it's the same with working for someone else, isn't it?). If you feel the income is good and the work atmosphere is relaxed, keep doing it. (She's not good to other relatives, not necessarily to you. If she's good to you, she'll give you more money! But you don't want to be too stupid to take a low salary and keep working for her)

If she treats you the same as other relatives, giving very little salary and doing a lot of things, then this kind of relatives don't want to, the sooner you leave her, the better.

Finally, if you go to work for your sister-in-law, even if she pays you well, you have to remember one thing: she is the boss, and you, like other coworkers, are just here to work! If any of your coworkers are making false accounts and stealing things and money from the store, you can tell your sister-in-law if you know about it. But if people are just whining, you don't tell your sister-in-law, or else the coworker gets kicked out and you'll regret it. (I worked for a relative for two years, this is my experience, I hope you keep in mind: you and your coworkers are just wage earners, do your own thing, don't betray others)