All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger. We ask Allaah to exalt his mention as well as that of his family and all his companions.
Your mother and your step-father have no portion in what your father offered you as a gift and they have no right to take any portion of it except with the agreement of everyone of you and while all of you having reached the age of puberty, or if your father had stated that she is included in this gift. It is permissible if what they had taken is against spending on you in food and clothing or what is related to it. They have the right to take from your personal wealth unless they had originally intended what they spent on you to be a donation. Khaleel Al-Maaliki said: 'If a person spends not intending charity and intending to take back whatever he spent, he can do so.'
Therefore, your mother and step-father have no right in your own money except if it is in exchange of providing for you. The money that the mother had given to your sister for marriage is effective as she is like a person in authority, and this was spent in order to achieve a benefit and according to the customs. If you agree to offer your mother and your step-father a portion of your wealth in return of their kindness and their nurturing for you, then this is something good. You will be rewarded and thanked for this act, and it is also considered as keeping good relations with kinfolk and consolidating relationships with relatives. All of this is something legally required.
Allaah knows best.