Hi OP NTA
I'm so sorry for your loss and hope one day the pain is not so raw.
Your sister is a piece of work, but after reading what your mother said I think we know where your sister gets the audacity to act this way.
First of all, regardless of whether you like each other or not, family is family. Your sister should have done something for you and your wife while fighting this battle.
Second, I admire your restraint at not slapping your sister at the funeral - beyond disgusting behavior. Then again, she seems to be consistent.
Third, approaching you at any time, but just at 6 months after losing your wife is again another display of horrid behavior.
Your mother backing her? Well I guess we know who the golden child is! I am so sorry. Your mother and sister ruined your mother's birthday. Good for you for walking out and taking the gift - let them have each other.
You deserve to be loved unconditionally by your family, I am sorry they are not there for you.
Stay away from them, maybe one group text or FB message if you use it - just say you are disappointed that your sister has been asking you for your wife's clothes since the day of the funeral and has called your wife a selfish bitch in death because she selected a charity for her clothes before she passed away - may we all be as selfish as your wife. Include that it is unfortunate that mother is supporting this criticism of your wife and trying to blame you for ruining her birthday. Since they can't act like decent human beings with a shred of moral compass you are taking a break from them and will not be in their presence, talk with them or be with any friends or family that want to talk about and support them. You are only accepting kindness in your life.
Leave them on blast and move on, you deserve better. Your true friends and family that care about you have already shown their colors while your wife was ill and when she passed away. You know who is reliable and trustworthy, stay close to them and away from others.
Don't defend yourself to anybody.
You mother and sister attacked your wife's character because you will not give your sister your late wife's belongings. WTF?
Donate when ready, in my family I've seen people need 3+ years before being ready to let go of belongings. When you are ready you will be ready, absolutely nobody should be talking to you about it beyond telling you if you need any help with anything around the house let them know.
Scorched earth to your mom and sister, they earned it.