My aunt and her husband have sent me a letter from there solicitor more or less saying if you don't let us see your daughter they will let out certain family matters what does that mean will they try to say Iam a bad mother

I have no idea what "let out family matters" means other than they are trying to blackmail you - they see the child OR they tell family secrets about you. Yes, anyone at any time can say you're a bad mother. Question is PROOF (or lack of PROOF).

There are no "automatic" visitation rights for aunts and uncles.

I would not allow myself to be blackmailed.