In Jainism, this would generally be seen as improper and lacking modesty, but not necessarily a gravest sin by itself.

A mother and daughter changing clothes in a private room is usually judged by:

• intention
• modesty
• whether there is any lust, disrespect, or wrong conduct

If it is only for changing clothes, with no sexual intent and no harm to anyone, it is not the gravest sin. But being naked together is not considered ideal conduct in Jain household life, because Jain dharma values lajja, maryada, and restraint.

So the simple answer is:

• Sin? Not a grave one by itself.
• Proper? No, it is generally considered improper.
• Gravest sin? No, not as described, unless there is lustful intent or other wrongdoing.

In Jain ethics, intention matters greatly. If you want to follow the most restrained and respectful conduct, changing separately is better.