Me and my ex know the truth so that's all that matters. It's really a matter between us. And at this point DNA is irrelevant
You keep saying that all you want is what best for your children, but all you do is fight against you ex's mother to get what you want and shoe little sign of doing what is actually best for the child.
There's clearly doubt over the paternity of your youngest. You say you were separated from your ex, from what you've written, it sounds like you had an affair, and the reason he wanted nothing to do with the youngest was because he didn't believe she was his.
Now it sounds like he is not sure and happy to play dad but with the option to step out at any time if it suits him. This suits you too.
Only his mother shoes some level of maturity and care for the child. A child who deserves to know whether the man who is kind of but not totally her father actually is. The right to have a relationship with her family members like her sister has, or if not, to grow to understand that this because they don't have the same dad.
You seem totally blind to understanding how this situation, lies (or whether you conveniently refer to 'private matters' affects both your children.
Being a good mum is much more than loving them and providing care. It is giving them the tools to grow a sense of identity that is healthy and giving them the chance to bond with other family members. You seem to believe that all they need is your life. You are heading for a very big shock when they get older if you continue with that attitude.