1. What sections are used for fighting "biased distribution from stepmom"
She is not forbidden by law to dispose her own property in the manner she may choose, you cannot object to it.
2. Does "injunction" suit also prevent biased distribution (such as gifting all to her biological grandson) within family?
Injunction suit against her own property is not maintainable.
3. If my father used his funds or mine to construct building in the said land - do father or i have any claim restricting sale or family distribution against our will?
The property standing on her name shall be her own property and this reason cannot restrict her from alienating the same.
4. My father is suffering from a chronic disease -- Parkinson's d. I plan to treat him in the United States with an expensive surgery (around $50,000 or 20 laks). Can I seek injunction in the courts seeking protection of movement of any their joint assets to fund the treatment
Your Father only can file a petition seeking injunction agaisnt his own properties.