If I change clothes in a private room with my mother is it's a sin we are naked so is it a grave sin or a faulty strictest answer possible no or yes

No — by itself, changing clothes privately with your mother is not a grave sin in Jainism.

In Jain ethics, intention and conduct matter most. If there is no lustful thought, no disrespect, and no improper action, then it is not treated as a serious sin. For householders, this is generally a practical private matter, not a grave wrongdoing.

In the strictest Jain sense, one should still maintain modesty, restraint, and purity of mind.