Hello
@mully and welcome to the Dementia Support Forum. I am so sorry to read about your sister, she was so young for such a diagnosis. You have certainly been through a lot recently between the loss of your husband and now looking after your sister. So it is no wonder that you sometimes feel that you wish you have not decided to look after your sister. First of all, please do not feel guilty, that is a perfectly natural feeling and many people on here will totally understand.
Do you have any help looking after your sister, if not please get into contact with your local adult social services to arrange a needs assessment for your sister and a carers assessment for yourself. It might also be a good idea to arrange a meeting with either AgeUK or your local CAB to check that you are receiving any benefits that you might be entitled to.
Finally, you might find it useful to start a regular thread in the - I care for a person with dementia - area. There you can ask questions, talk about how you are feeling or just let off steam. I have attached a link below.
This forum is for people who are involved in the care of a person with dementia.
forum.alzheimers.org.uk