I've recently been informed that one my family members is terminally ill, and is only expected to survive 12-36 months depending on treatment.
When should I inform my employer? What are my risks?
If it might affect your work performance, out if you might need to take time off with little advance notice, or if there is any other reason they should know... Why would you not tell your manager?
Right now. The sooner they know the better. Even if it doesn't affect your work at the moment give them some head start so they can plan accordingly.
You might find out that your employer thinks that the work you do for him is more important than family and your wellbeing. In which case you might want to reconsider working for them anyway.
A reasonable employer will be understanding and cut you some slack until things are resolved.
All the best to you and your family!
As soon as you can.
There are no drawbacks, you may need to take leave suddenly with little notice, and the sooner your employer is aware of this the better. Everything else is secondary to that.
There are drawbacks to telling them later, such as leaving them in a bad situation workwise. because they had no idea you had urgent personal business to attend to so they had no time to make a fallback plan.
I'm very sorry for your situation, that is a tough thing to go through. Before your engage your employer, please spend some time thinking through the situation: How it will likely develop and how it will affect you personally and professionally? How will it affect your ability to work (both mentally and physically) and what's the best way to structure your work environment so your needs can be accommodated.
A few examples:
While this is very painful, it's unfortunately not that unusual. Many employers have experienced this before and will be try to accommodate you within reason. However it's up to you to determine what accommodation you want and need. This is very personal and every person and situation is different. You shouldn't expect your employer to figure this out.
So the recommended action would be: figure out what you want your employer to do and then go and ask for it as soon as you know. As long as you ask respectfully and don't demand, there is no risk at all, unless you have a terrible employer.
One final advice (from personal experiences and both sides): This time frame is long enough that it's important to establish some "Life goes on" attitude. While this may sound unfeeling, it's vital for all people involved. Putting your life (personal, professional, family) on hold and just waiting for the inevitable just drags everyone done, including the person that's dying. It's okay to go on vacations, it's okay to go to parties, it's okay to start planning that wedding, etc. Have a good time, tell your loved one about and he/she will have a little bit of a good time too.