There are no rights, not even a retained right of residence, for unmarried partners of EEA nationals who are victims of domestic violence under the EEA Regulations.
Retained right of residence for victims of domestic violence under the EEA regulations is applicable only to spouses and civil partners (See Page 18 of the
Retained Right of Residence guidance).
I am not sure that your sister can apply on
Form SET(DV) as she does not currently hold leave under the Immigration Rules.
In theory, if her EEA partner informs the Home Office that they are no longer in a relationship, her right to reside in the UK comes to an end. She is only eligible for PR if her partner agrees that they are still in a relationship when she makes an application for PR.
Her only option in the near future seems to be a
Zambrano application (assuming that the child has inherited Swiss citizenship from the father;
Swiss nationality does not descent automatically from unmarried Swiss fathers), but that is a derivative rights based application and does not lead to PR.
I am not a lawyer or immigration advisor. My statements/comments do not constitute legal advice. E&OE. Please do not PM me for advice.