The number of people supporting what this relative did (and even confessing they did the same thing when their child was little) is very concerning.
A 2 year olds brain does not work like that. All they are learning is this adult is not safe, it’s okay for people to hurt me, my mummy and daddy will not advocate for me if I am hurt.
This is assault and is cruel what your relative did. If you tell your GP I believe they will have to make a safeguarding report and Social Services may get involved. I strongly think you should as this needs to be recorded.
What has your husband done about it? Especially as his side of the family.
Do not leave your child alone with this person again. Or anyone in the family that supported their behaviour. Send a strongly worded email to them and all involved that it was a serious breach of trust, they should have come to you, and any further violence towards your child will
be reported to the police. You HAVE to start advocating for your child, even if your husband won’t.
Also, re biting sibling, try to figure out why it is happening: are they trying to get siblings or your attention, get a siblings toy, is child teething or anything else. Then help them meet that need in a more functional way - using words or sign to ask for the toy or practicing turn taking, asking for hugs etc to get siblings attention, chewies or crunchy foods to meet any sensory needs etc.
Punishment is less effective than getting to the bottom of why the behaviour is occurring in the first place.