You ABU.
While the behaviours of individual autistic children vary greatly, telling outright lies falls into a category of ‘generally not what they do.’ They might sometimes use words that are stronger than the situation warrants (what is normally considered exaggeration, but in the case of children with autism, it may be because of vocabulary limitations, or perhaps otherwise because their perception of an event requires stronger language than another person might use) but it’s unlikely that the autistic child is telling lies about your daughter and her friend.
Few parents want to think their child might choose be unkind to other children, nor to believe that their child might be the instigator of unkindness towards or the bullying of another child, but the fact remains that children are unkind to each other, they do form groups or pairs and bully or otherwise target other children, and regardless of what the teacher said about your other children (no doubt in an exhausted, end-of-term attempt to arouse some empathy for the autistic child), you need to accept that it is likely that your daughter and her friend have been unkind, and that you need to act on it.
There are no excuses for unkind behaviour.
That applies to parents as well as children and teachers.
This is an opportunity for you to practise some empathy & kindness, both to the teacher (cut her a break) and to the child who has, quite rightly, told the teacher about what was making her unhappy. The teacher probably realised that she shouldn’t have mentioned your other child’s disability, nor that of your daughter’s friend’s sibling, as soon as the words left her mouth.
Let it go. If it happens again, whether privately or in front of the class, then call her to have a chat about it, but it could well just be a one-off. It might be helpful to have a chat with your daughter and to reflect together on whether a disability on the family needs to be a private shame rather than something that can be openly talked about.
Teach your daughter to be her best self. She doesn’t have to like everyone, but she does need to be pleasant and respectful towards everyone, and kind when the situation warrants it. Explore with her the thoughts & feelings that trigger unkindness, but do not allow them as excuses: help her to find better ways of managing those feelings, and check in with her regularly (I’d say at least thrice weekly) about how she managed a situation that wasn’t her choice or preference at school (e.g. if she was put in a group or a pair with someone she didn’t know well or didn’t like for some reason).
I know it’s hard. I wish you luck.