I want to marry the daughter of my mother-in-law's younger sister? She’s married and she has a kid. But her husband don't take care them properly. And I want to give a good life to both of them.
You can't marry a married lady as this is simply haraam. You can't help (or make) a married lady to divorce her husband as this is a big sin, even worse if you want to marry her afterwards:
The Prophet (ﷺ) said: Anyone who incites a woman against her husband or a slave against his master is not one of us. (Sunan abi Dawod)
She must get divorced (on her own will) and finish her 'iddah before you may do anything.