Well I didn’t want to get into detail but you still seem concerned.
To be even considered for custody the offence must fall into Category 1. (Even then, custody for a first time offender, especially one pleading guilty is extremely unlikely). But anyway, to fall into Cat 1 there must be “greater harm” and “higher culpability”.
If you look at the guidelines, from what you say the only factor which could make the offence one of “greater harm” is the vulnerability of the victim (a youngster). There are no factors that particularly indicate higher culpability. There seems to be no element of planning or targetting, nor of a sustained attack. At the very worst the offence might squeeze into Category 2 (because of the victim’s age) but that’s about it. And as you can see, the guidelines for Category 2 do not include custody as a consideration.
Magistrates must sentence in accordance with the guidelines, basing their decision on the facts presented to them. Of course it’s a worrying time but I think you can be assured, based on what you have told us, that custody will not be considered. I should also point out that unless custody is justified by the sentencing guidelines (i.e. it must be in category 1) then a suspended prison sentence cannot be considered.
My view, based solely on what you have said (which is nowhere near as much as will be presented in court) is the court may well feel able to deal with this by way of a Conditional Discharge. Your sister’s previous good character, the element of provocation that was present (i.e. the threats to the eight year old) and her personal circumstances (bereavement, etc.) may well see that as a possibility.