Does one wife have the right to be treated as good as another wife of one husband?
What happens when she is not?
Is there anything she can do? What?
Yes, each wife has the right to be treated equally as good the other wife of one husband.
Polygamy in Islam is based on treating all wives equally and with justice as per Quran. Else the man is asked to marry only one.
O mankind, fear your Lord, who created you from one soul and created from it its mate and dispersed from both of them many men and women. And fear Allah, through whom you ask one another, and the wombs. Indeed Allah is ever, over you, an Observer. And give to the orphans their properties and do not substitute the defective [of your own] for the good [of theirs]. And do not consume their properties into your own. Indeed, that is ever a great sin.And if you fear that you will not deal justly with the orphan girls, then marry those that please you of [other] women, two or three or four. But if you fear that you will not be just, then [marry only] one or those your right hand possesses. That is more suitable that you may not incline [to injustice].
If she feels that she is not being treated well:
1. She should first speak to her husband and try to sort the matter.
2. If that doesn't work, she should involve her parents/close ones to make the husband understand the issue and sort out any misunderstandings or ill givings from both sides.
3. The last resort is divorce which though Halal is not something beloved to Allah.
As mentioned earlier, an online forum isn't exactly the best place to resolve family issues. I wish you the best and inshallah all will work out for the best in your life.