It's been 5 months now my sister didn't have her period. She has two negative pregnancy tests... it's really hard for her to go to the doctors due to their financial crisis. I'm really worried about her!!

I know you said its hard for her to go to the doctor, but she needs to. She needs to discuss with a doctor what is going on, and he may possibly want to prescribe her some birth control pills to regulate her period. She also needs to have a quantative blood test done to confirm she is not pregnant. Home pregnancy tests are not always accurate, and some women go their whole pregnancy without ever having a positive reading on an HPT.

The county she lives in should be able to give her aid on medical bills if she does not have insurance and cannot afford the bills. :)

I would like to add, if she is not pregnant it sounds like she has something called Amenorrhea, which is lack of a period.

Here is a website about Amenorrhea and the causes and treatments : No Period But Not Pregnant: Secondary Amenorrhea

She needs to go to a doctor ASAP. I don't want to give you reason to worry, but PCOS, ovarian cancer, tumors, cysts, early onset menopause, stress, minimized ovarian function, iron or vitamin deficiency and several other factors could be the root of the problem.

I can understand fiancial difficulties because I was diagnosed with ovarian cancer when I was 19 and I didn't have any insurance. I still went to be taken care of... even though I was afraid of the expense and lived with my symptoms for over 1 year before I went. Good thing, otherwise I wouldn't be here today. Health has to be her #1 Priority.

Hopefully, its no big deal.

Not having a period could be caused by different things. If she had regular periods up until when her period stopped 5 months ago, it could be amenorrhea.
Severe emotional stress could also cause a period to stop. Diabetes and certain medications can also stop a period. Bottom line, there are plenty of reasons her period could have stopped coming and either way, she should seek medical help immediately.

I know you mentioned the financial situation isn't so good, and here's my solution for that:

Go to the ER. They won't turn you away due to lack of insurance and she won't have to pay up front. They send her a bill after she's been treated and she can make arrangements to pay it off little by little.

Be advised: I work in a hospital and I can tell you this as a fact~ If she does have some insurance, they will most likely refuse to pay their part if she goes to an ER and its NOT an emergency (which I doubt this is, unless she has severe pain or other symptoms besides a loss of menstration)
Also, because its an ER, they tend to charge patients the maximum amount allowable for any service they perform. Plus, not being an emergency, they'll make her wait. Unless its an emergency, its wise not to use the ER... you will spend much more $$ in the long run and to be honest, healthcare (insurance) expenses are going up because people use the ER to avoid paying bills. So the insurance company will penalize you and the state might deny any support for "mis-use" of medical operations.

An "emergent care clinic" or a regular clinic will treat her quickly and without insurance... plus they're easier and cheaper to deal with. If she has an existing OB/GYN ask her to call and explain that she'll need to set up a payment plan. Many doctors will still accept you with no insurance (especially established patients) as long as you tell them you have none and they'll give you a co-pay amount due at arrival... the rest would be billed.
GL!

May seem like I'm asking an odd question... but how old is your sister.. and have her periods been regular till now.. age and weather or not you are active in your daily life can be a cause of delayed periods.. stress and worry can also bring this about.. but to relieve any worrys I'd tell her to see her physician.. asap