Is permissible for me (a son in-law) to help his ill father in-law get dressed and undressed?
Is his daughter able to help him under similar situation if I am not able too?
Both are fine. Although a son-in-law is obligated to some degree in the honor of his father-in-law (we have addressed the issue elsewhere), one of the obligations of honor is to “dress him” (one’s father), and if you can do this in a literal sense, it is certainly a way of honoring him.
It is likewise permitted for a daughter to dress him. In this case, seeing him unclothed is not degrading, but rather an act of honor in helping him to get dressed and undressed.
An explicit source for this is the halachah of going into the bathhouse with one’s father (Pesachim 51). Although it is forbidden to go into a bathhouse with one’s father, because one should not see him unclothed, it is permitted to do so if one’s father requires assistance.
This is ruled by the Tur and the Shulchan Aruch (Yoreh De’ah 242:16), and the Beis Yosef adds that although the Rambam doesn’t mention the halachah, he certainly agrees with it, and doesn’t mention it because it is obvious.