You are the only child of your mother's marriage to your father?
Your 'sister' and 'brother' are your mother's stepchildren by virtue of a previous marriage to their father?
Your father is deceased and your mother sadly died recently?
If you have answered yes to the above questions , you are therefore your mother's next of kin in law and, in the absence of any will or signed document to the contrary, your mother's estate including all of her personal goods and chattels passes to you in its entirety and you are under no obligation whatsoever to share or otherwise give anything that previously belonged to your mother or father to any other relatives.
In this instance, I am firmly of the opinion that the items of jewellery that your mother most cherished including, but not limited to, her own and her grandmother's wedding ring and the diamond ring she was given to celebrate her 50th birthday, should be retained by you and handed down as heirloom pieces to your dds at a time of your choosing.
Unfortunately, at times of bereavement it's not uncommon for greed in some form or other to rear its ugly head and I suggest you tell your stepbrother, stepsil and any other party who is crass enough to impose on your grief that you intend to defer any decision as to disposal of your mother's jewellery collection until a later date.
I am so very sorry for your loss and I sincerely hope that you will not rush, or allow yourself to be pressured, into making any decisions that you may later have cause to regret.