Need serious advice, my mum wants to destroy my marriage

Married with four kids. My mum told me when I was 20

years(1974) that my real dad was different from those of my sibblings. She said that her

present husband, wasn't my dad. It was not a good story.The story did not devastate me so much then because I just found my present husband few months before.

My sister came to live with us in 1997, spent seven years and left us in 2005. We went

through hell while she was with us. Lying, colliding us with neighbours, stubbornness and

flirting. We did all within us so that she could preserve herself, but she broke out of

control in 2004, when she started flirting. We sent her away in 2005 to the parents in

another town. Before she left, she gave us a note where she confessed a long list of the

people she flirted with and the number of times each.

She got pregnant in August of 2006, precisely eight months after leaving us. My mum called

and informed us. We informed her to tell her husband so that they can find who pregnated

her. She was 24 years then.The dad threatened her. She brought a boy who she said was

responsible. The dad made her to marry him. When she gave birth, they parents of the boy,

knowing her background, decided that their son should go for a DNA test. The test proved

that he was not responsible for the child. Hence she was divorced.

Back at her parents' house, she decided to start saying nasty things about us. She said we

were so strict, that was why she spoilt. My husbands' enemies heard it. They deceived my mum and told her to join up with my sister and spread that it was my husband who pregnated her.

The so-called person who decieved them is close to my mum.He called meetings of different

people to spread the story to. When the story got to us, we asked them to come for a DNA

test. For a year and 8 months now, they havn't showed up.

Now my younger brother(31 years)came to this town we are in, and is showing people the

picture of the child and telling the same story. They have since convinced my so-called dad,

who is also spreading the story. Recently, he sent a text message to a family friend,who

forwarded the text to us, detailing his name and who he is and those terrible things they

claim my husband did. The man who started all this died in a terrible accident on May 18th

this year.My mum told people that she wants to destroy our marriage with this. We want to

take a legal action. how do we go about it? How do we stop this problem? how do we stop this

scandal? Thanks in advance.