Is a girl sinful if she does not say "Amen" when someone supplicates for her to have a righteous husband, out of shyness and a desire to conceal her wish for marriage, while she herself consistently supplicates for that?

The one who says "Ameen" to someone else's supplication is considered like the one who supplicates. So, if someone supplicates for you to have a righteous husband, and you do not say "Ameen" to their supplication for the reason mentioned, there is no blame upon you. It is not obligatory for you to say "Ameen" to their supplication.