Praise be to Allah.
From the question, it seems that the questioner caused her mother’s death by mistake, because of her misjudgment whilst overtaking. Moreover, if it is true that the other driver deliberately increased his speed, which led to the accident happening, then what appears to be the case is that the blame for killing is shared between both of you [you and the driver of the truck]. In such cases the matter should be referred to experts. If they determine that the accident was caused by the mistake in overtaking, then you must pay the diyah (blood money) in full and offer expiation. If they determine that the blame is to be shared, then you must offer expiation and part of the diyah, commensurate with the extent to which you were to blame.
For example, if the specialists determine that the blame was 50% yours and 50% that of the truck driver, then you must pay half of the diyah, and if they determine that the blame was fully yours and that what you say is not to be accepted, then you must pay the diyah in full. For more information, please see the answer to question no. 85426.
The scholars differed with regard to the expiatory fast and whether it is to be done immediately or may be deferred. But undoubtedly to be on the safe side and to discharge one’s duty, it is better to hasten to do the expiatory fast when one is able to and there is nothing to prevent one from doing it.
For more information, please see the answer to question no. 130703.
If it so happens that the days when you are going to fast will include the days of the two Eids or the days of at-tashreeq, which are the three days following Eid al-Adha, and not just two days as mentioned in the question, then breaking the fast on these days will not spoil the continuity which is required for this expiation, because there is a legitimate shar‘i excuse.
For more information, please see the answer to question no. 124817
And Allah knows best.