I have cared for my 85 year old mother in my home for the last 5 years without compensation. Can she reimburse me now for the past years?

As Igloo says, without taking some legal action now, the retroactive payment will be seen as a gift. It would be worthwhile to see an Elder Law attorney about this matter. The cost of the consultation will be small compared to the possible consequences of doing it "wrong."

I wish ALL caregivers would arrange and document reasonable compensation from their loved ones from the beginning. I am very glad that my sister is accepting money from our mother who is living with her.

Does mom have the funds to do private pay for NH for the next 5 years? If so, then she can do whatever with her $ and assets. But if you are going to need to apply for Medicaid, there will be a 5 year lookback possible on all her financials.
When you apply for Medicaid, you sign off on an all access pass to all their financials, stuff will eventually surface. For my mom's application, the required banking documentation was for 3 years & 6 months plus front & back on all her insurance policies, funeral & burial policies, assessors statement on property ownership, etc. It's a pretty detailed review and moving $ around at the end will be found out and cause the application to be denied unless the $ cab be justified with documentation.

If you have her pay you now, the reinbursement will be viewed as gifting and will cause a transfer penalty issue to coming into the picture for Medicaid. Caregiving by family without a valid personal services contract (with all the 1099 & tax consequences) seems to be viewed as done for free as a loving familial duty. There could be the possibility of having the elder care attorney do a personal services contract and other documents regarding her room & board now, but really you need good legal to do it correctly so that there is no transfer penalty issues.