property problem

(I am not disclosing any names just expressing them with a,b,c letters) My elder sister (a) is married to (b) a sbi employee and they have a 3 years old daughter(c) and in their family (b) have a elder brother(d),a private company's employee who is married with (e) and they have a son(f) of 1 year old, and their parents are in their family.Now they have total 3 bedroom,1 bathroom and 1 kitchen that means one for their parents , one bedroom for (a,b,c) and the 3rd one for (d,e,f). so very congested situation,now (b & a) that means my brother in law and my elder sister wants separation in property, at this moment the property owners are their parents, but the parents are neither willing to have separation nor giving any permission to (b) to builds any construction for extra bathroom and a study room for their daughter.they are saying just 'if you want to build,do it but there is no assurance that on the property_will that very bathroom and the study room will be for (a,b,c) only' and more over their parents are not giving any hint that they will give them a single portion of the property.Whenever (b) asked for the property separation,his parents are saying if you don't want to live jointly then go live somewhere else.actually my sister &my brother in law don't have any problem in living jointly but problem is giving money for his brother's family purpose that means for (d,e,f), And (d) is not giving enough portion of money for family purpose, in reason they are saying (d) earns less than (b).So (b,a) wants a separate property cause they don't want to take his brother's family's financial responsibility.(b) also suggest his brother to give same amount of money or at least same ratio of money but his elder brother always give some excuse to avoid to give same ratio or same amount of money as much as (b) gives for the whole family. now my questions are- 1.can (b) asked their parents to have separation of property? 2.can (b) build a single extra room on that property? if yes, then who will be the owner of that extra construction? and if the owner becomes their parents then can (b) claim it back, if their parents don't give a single portion of that property to (b) ? 3. Can their parents make legatee their whole property to only to their elder son (d) ? is there no right of (b) and his family to get a single portion of that property? please help me with the answers to these question and suggest me how can I help my sister and her family, and any suggestion for my sister's family will be welcomed. thank you. Uddipta sen