How do I ask my estranged sister to be guardian for my son if I die? They are close.

Start conversation. If you & sister have been estranged for long time, start to re-establish ties by accepting her as she is and agreeing to disagree on whatever issues you have. You can't change her and she won't change you, but if you are asking her to care for your son if you die, it can help make a traumatic change less awful for your son if he can see that you & sister have mended relationship somewhat.