Husband and Sister! 😡😳

Ok I am going to lay all this out!! I need to make sure this is not all in my head and need advice!! 
So I've been with my man for about 10 years! We have three children together. Yes he has cheated on me when I was preggo with my twins and then got the other girl preggo too! I eventually took him back and forgave him..So now my older sister has three kids of her own and IS married but separated but lives with the boyfriend. Now my sister and her boyfriend are clearly on meth or drugs of some sort bc they look horrible!! Now to the weird part!! So she's finally got a new phone and the first person she texts is MY MAN! Why?? You've already come out and told me he's bought you things and given you money..now she is bipolar!! So I see this text from MY sister on his phone and I said why is such and such texting you and I read the messages and it's like hey this is your sister and all this mess..well NOWHERE in the message does it say make sure you give (me) my number so she'll have it...NOTHING! So then my man comes and says something strange that sticks with me! I didn't go to work Saturday because I am sick, and so that's usually when she comes over my house is on the weekends and she texted him and he said oh I didn't tell her you were home!? Why would it matter if I'm home or not!?? That's my sister? They've been acting weird and I've vented to her about him and she'll either agree with me or justify him. Then last night she called his phone and so I answered it! My man was on the couch and I said hello and she said oh hey and I'm like hey and she said I called to talk to you I can't remember your number!! But she can clearly remember my mans number!? Is this strange and should I dig deeper?! I've even thought about buying a nanny cam for when she comes over and I'm at work!! Or is this all in my head?? Help!!Â