i would suggest getting a second opinion and seeing if the other lawyer says the same and has the same idea of how he or she could help ensure your success.
as others have mentioned, your sister has a husband. I get the family bond and i am sure whoever is assigned to review your case should you pursue it will also feel for you but the root of the matter is, is it enough to grant you PR? The visa officer will need to determine if you getting permanent residency is absolutely needed. If your sister is terminal and the reason you want to stay is to be with her until she passes, why should you be granted PERMANENT residence? Please note this is in no way an attack on you but the logical train of thought any visa officer will have. You are not needed in Canada permanently, this is a fact. Your sister has immediate family in Canada since the husband and children are here, this is a fact. I don’t know you nor the husband and i don’t know the actual facts to your claims that he is being neglectful. Ensure you have proof ready for that also should you decide to file an H&C app.