

Make sure to use vocabulary they understand, but don’t be condescending with it. They’d rather you speak to them like they’re older than they are than younger.
Don’t be overly restrictive of their freedom, but be clear about the limits and make sure to enforce them. Explain the reasons for the restrictions rather than just telling them they’re not allowed and they’ll generally listen.
Give them small jobs to make them feel more responsible and patient about other things.
Do your best to answer their questions properly instead of dismissing them, even if the only answer you can give is “I don’t know”. They’ll listen to adults that they feel listen to them.
New answer for the specific context:
You did your best with the 2 year old. Toddlers are difficult because they just make noises and expect you to work out what’s wrong for them. Small snack and something to drink are always good starting points, followed by a distraction (like the nursery rhymes video) to get their attention off what was upsetting them.
With the 5 year old, you escalated by increasing the punishment to 15 minutes, and now your brother-in-law pushed it back down she knows you can be overruled. Talk to your sister and her husband to make sure you’re both clear on what limits your comfortable with and what specific punishments to give for breaching them, in line with what they’d normally give, so you can both be sure you’re giving out punishments they feel are appropriate, so there’s no room for her to get you overruled - if she goes to her dad you should both be confident he will back you up. She’s of an age where she is going to test what rules and limitations really exist, so you need to make sure she’s not getting mixed messages about it. You’ll have an easier time distracting her with something interesting than telling her not to do stuff.