Help with a sister in law who has bi-polar!

My sister in-law has bi-polar disorder and OCD. I love her dearly and so do my family but when she is in her low point from her bi-polar it is really hard to know what to do. When she is up she is really great and fantastic but when she is down she retreats from us for sometimes weeks at a time and when she does talk to us she is really snippy and we cant say anything right! I know she cant help it and she has suffered some major trauma as a child too which doesnt help, but does anyone else have a friend or loved one who has bi-polar? How do you support them at their lowest?