I'll try to my best to answer this in a structural and simple understanding, insyaAllah. I did so after some googling of articles on my local Islamic body site.
Other's may correct me if I'm wrong. If there's any lack of accuracy in the info is my fault, the good comes from Allah
Well, basically Muslim's women are mahram to each other...but other than that there are few categories as listed below:
1. Mahram based on ties of blood (Nasab)
- Her grandparents and above (like great grandparents and before them)
- Birth parents
- Uncles/aunts (exclude their male spouses )from both father/mother's side and their children and grandchildren and so on (nephews, nieces)
- Her own children and grandchildren and afterwards
2. Mahram by marriage
- Her in-laws and above (in-laws parents, grandparents and before them)
- Son & Daughter in-laws
- Stepfather (after he had married and consummate with her wife of which is the woman's mother)
- Stepson and below (only after the woman had married and consummate with the son's father)
3. Mahram By Breastfeeding or Rada
- The breast milk mother and her husband to the fed child
- The breast milk mother's and husband's parents to the fed child.
- The breast milk mother's and husband's siblings to the fed child
- The breast milk mother's children and after to the fed child
- The fed son/daughter's and their children and after only (the child's blood father and uncles and before them are not mahram to the breast milk mother)
the requirement to be a mahram by Rada/Breast milk fed are:
* The woman is capable of producing breast milk and baligh.
* has fed the baby either by her own breast or bottle or any method to share the milk (like spoon or fingers)
* the baby must be below 2 yrs.
* The child must have been given the body milk and is fed separately a minimum of 5 times.
* The milk must be consumed by the baby that it becomes the food and not for other reason like for bathing and other external purpose.
Hope it helps ya!