My Mums having an affair.....Help?

I think my mums having an affair,shes had one before and just expects to be forgiven if caught out.I love her but she is really selfish! My dad is a good person and he doesnt deserve this.They have been married for bout 15 years.I dont know if she is bored or something?she pretends she loves him to his face but i know she doesnt.He's obviously oblivious.Right well now - My mum has been texting this guy,they say all sorts of kinky things to each other and they have weird nicknames They send pictures to each other and all that.Once i went on her laptop and there was - I will be a good girl and not piss off my master.(She calls him the master on her phone) She is EXTREMLY protectiv eof her phone to avoid detection but when she is drunk i have a wee look.She also said - If i didnt have kids then i would run away,And she told him shes had an affair before.Me and my sister are worried and not sure what to do !!!!!!