1) If you're estranged, bury the hatchet. It's a chance to clean your lenses. She's the mother of your kids and if anything happens to her, your children will be devastated. Do it for them.
2) Her emotional well-being is not your problem, but it is your concern. Call her every day to ask how she is. See if there is anything you can do for her. Little, tiny things like a surprise bag of clementines will let her know you care.
3) Do the little things requested of you. It might be running an errand or picking the kids up at a time that makes her life easier. Most of the time she'll say no, she's good, but she'll feel loved and supported like the family member she used to be and in some way, still is.
4) Give here extra money if you can. Maybe she's not "entitled" to anything and you feel soaked, but still, slip her a few bucks so that she might buy something extra for herself or the kids. It'll be a gift to yourself. You'll see.
5) Step up and be present with your kids. Listen. Give them space to share. Give them time with you in silence if that's their style. Regardless, give them a hug. Every day.
6) The laws of impermanence are irrefutable. We're all going to die one day, and if you feel they're old enough for the conversation, tell your kids that this is an opportunity to learn to practice immediacy and gratitude for family, friends, community and beyond. There's no guaranty in life, so everyone in the family can learn to forgive and be loving right now. Ask your kids consider this lesson -- maybe even thanking their mom for her bravery in walking this path -- for teaching them the meaning of gratitude and being alive now.
7) If she doesn't have a partner and lives alone, bring a meal or offer to do some heavy lifting chores.
8) If your current partner objects to your concern for your ex, remind her that once you love someone, it's counterintuitive that you'd stop caring for them in a time of need. Cancer strips us bare. The essence of love remains even if it has morphed into a fraternal love reserved for a beloved sister (or an ornery aunt).
9) Get a little extra involved with the kids. Talk to their teachers and school counselor. Do stuff with them. Check out resources like the book My Parent Has Cancer and It Really Sucks.
10) You don't have to paint her house. Just be nice.