Redecorate gradually. She has done it upstairs which isn’t too disastrous. I’d be more worried about her painting on your wardrobe?! How on earth did you get it off? And how did she have time to gather all the supplies and do it all without anyone noticing?
I would take this as a cry for help as it is quite extreme behaviour.
Withdrawal of iPad/holiday, naughty step etc seems a sensible way of underlining the gravity of what she has done. A shame DH is not more supportive.
If she is ND then her mental capacity in some areas may be more akin to a toddler and you need to treat her as such.
I would suspect issues have escalated from the birth of her brother when she will have received less support (naturally). It’s now coming to a crisis point.
She needs strong boundaries, routine, visual schedules, visual rules, emotional regulation techniques all of that.
Read up as much as you can, when you can, and reach out for help and support.
Lots available online, and would recommend this book for overview.
—> Women and Girls with Autism Spectrum Disorder: Understanding Life Experiences from Early Childhood to Old Age, by Sarah Hendrickx
I wouldn’t worry about the walls.
If she is ND it is a blessing you are finding out now, rather than she goes through school/Uni/work without support, and ends up being taken advantage of/assaulted or worse due to her vulnerabilities, or being put through the mental health merry go round unnecessarily.
If she is not ND this is a pretty startling wake up call that her needs are not being met. She may need more discipline, time with you, other activities to stimulate her or issues in school addressed.
Let us know how you get on.