Is it permissible for a Shia girl, who has converted from Sunni Islam, to marry a Sunni boy solely for the sake of her mother's happiness, despite her awareness of the truth of her belief?
Marriage between Muslims from any Islamic sect is permissible in principle, but if you know that this marriage can harm your faith or religious practice, then you should avoid it because saving your faith and religious practice is more important than marriage. Your mother's happiness can not be through endangering your future. You should explain to her in suitable and polite way your situation and never allow such forced marriage if you feel that it can harm your faith or religious practice.
'Wassalam.