I have started getting suspicious that my husband is having secret sms and mobile conversations with my sister?

he has lost interest in me no longer loving and affectionate as he used to be...she is a divorcee with a young son..she sends sms ...missing u so so much....but writes my name so i dont guess..i hv now seen hubby expression on his face when he chats to her...or when he sms her he makes long sms not short ones...and so preoccupied in writing them... and he goes over to see her more often than me ..my mum dad ands sister live together..he uses my mum name that she inviting me for dinner , tea or other excuse and he gets excited to go there, my sister cooks special food cakes sweet dishes for him everytime....im so sure its been going on for years now that she has eyes on my husband and favours him...and went to divorce her husband becoz she been comparing to my husband...my husband started complaining in bed that i take such a long time to orgasm....his mind is elsewhere all day and he spends little time with me and gets cross quickly if i have a debate with him about any little matter hewalks out of the house and leaves me alone...then he walks past me as if im invisible....these are the evidence i have ...i live abroad with my husband and he takes trips regularly to home where my family live becoz im not keen on travelling...i hv been trusting blindly my husband and not till today would i hv ever guessed such a horrible thought...but since recently her birthday came my hubby rushed to sms her and she rushed to reply back to him and he immediately sms her that we are singing happy birthday to you ..then he put the mobile away on one side ..not showing me although i saw from nearby what he was busy doing...he is using our own son as scapegoat...now my sister is using my son to agree to come back home not to live abroad ... when my hubby goes to my mum and sister he involves himself so much in my sisters son and she keeps photos in her mobile and sent me email also to show...maybe to make me jealous....i remember i mentioned to her i was having marital problem and that i may seek a divorce.. my sister replied yes well whatever you want do it...i thought that was an odd reply at the time i didnt realise i brushed it aside..but now its nagging me day night that this affair has been going on while he been making visits and i been abroad with my son....what do i do and how do i handle this problem...im unhappy and im surprised this been going on under my nose and its been a secret...maybe my mum knows too becoz i complained so much about my husband but she just laughs it off and says leave him if thats your decision...???? my mother adores her son in law and showers him with love and money and comfort....i get furious becoz i feel my mum and sister are overdoing their share...but i cannot keep my husband away from them becoz they keep on contacting him on mobile and he drops everything and runs to them like lightening....he just mumbles and shrugs it off that they are just calling me for a chat or food thats all......plz help.