Have feelings for my mother in law

I've been in a 6 year relationship with my gf/ childs mother. We plan on getting married in a few years. My lover is the only child and is very attached to her mom. My and my childs mother met threw smoking weed. Lol yes young n dumb. Over the 6 years we made 3 beautiful daughters. My gf isn't like most woman and I realized this within the first year of dating. She's not the cooking type nor does she tend to pick up behind herself. Yes!!! Some of us men do pick up the slack without caring for a reward. So during our dating her moms been somewhat into her own life. She's had numerous men that never amount to shit. Over the past year we decided to all live together because its cheaper, we would be able to buy more and spend more on each other and so on. I tho ok first time for everything right!!? During the first year she was dating this drunk that did nothing but spend money and tap into her funds to support his habits. After seeing how reckless he was living i spoke up to mil. She literally told me she wasn't happy and she was getting a divorce. Ok all good "he just wasnt the one". My mil is 60 she's def a hard worker and well she's not really wasn't attractive to me until our bond was forced upon us. How? Well my gf knew I had mother issue and we talked about all our problems during that first year of dating. We accepted each other flaws and created the family we have now. So during that first year "after my mil filed for divorce" my gf would force me to spend time with her mom because we both knew she was going into this depression stage and talked about losing herself somewhere along the way. Knowing Im the man of the house i decided to cheer her up and tell her that happiness lys withing yourself first before sharing with others. She's a spiritual woman who's just had enough of dating liars and bad men all her life. So one day we was alone and talking. She told me about every horrible guy that walked into her life. I couldn't believed that she's had nothing but horrible relationships. I could see the tears forming in her eyes so before she could shed a tear i got up and walked over to her and gave her a big hug. Not knowing i had morning wood still and i could feel my dick "9 inches" hard af on her stomach. I was embarrassed because the timing and everything was so freaking off and i had morning wood. Over a few months I've noticed she loved to hug me. No biggie right? Well my mil is into bdsm and her last husband dick didn't work so she hasn't had good dick in decades. She literally informed me of this. I comforted my gf because i don't keep secrets. My gf was shocked but at the same time laughed and asked if i felt some type of way about her mom. I said no but i do love her like my own. One day my gf came into the room crying. I instantly wanted to know who made her cry because boy somebody was going to get that face punched in. Well it turns out she was sad for her mom and been talking to her for the past hour when she got off work. She informed me about how her mom is lonely and how its crazy she hasn't experienced 1 good man in her life. So i asked her what do you want me to do about this. Before she could say anything i said " i could flirt with her and make her feel like a woman" my gf laughed and said yes. That she would do anything to make sure her mom didnt go down that depression road again. So for months i catered to both my gf and my mil as if she was my own gf. My mil doesn't want to form another relationship because she feels she's getting to old and she has herd several times me pounded the hell out of her daughter. Now all of this is new to me and I've never lived in a home with two woman. So yes they both at times seek my attention at the wrong time lol but i do what i do best. Well just last week we had a issue and my mil started to dislike me. She would literally try to get me and my gf to argue with hoping my gf would dump me. Note im the dominate one in the relationship and im one kick ass bf. My gf spoke up and demanded her mom show me respect and treat me like a human. I don't want to go into details about what's she's done but lets just say she made me feel like a slave. Yes im black and my gf is white. So her mom at times lay her life on me then pulls back. I know my job is to be a good son in law but i can't help that both have put me in a situation to where i love her mom as if she was my own gf. I even spoke up and told my gf. I told her everything and yow i felt. My gf wasn't mad and all she said was as long as its not another woman i don't care. My mil always flirts with me, some times she would come to the breakfast table with just her robe on and i could literally see her nibbles and the color bra she was wearing. My gf never gets offended and just act as if this was all ok. So would it be wrong if i fucked my mil knowing i could please her better than shes ever felt before and knowing my gf doesn't care but a part of me says she cares a little. We both know her mon just needs good to chill that frustration out at times and its crazy how she choose no to go get a fuck buddy her age or just have men friends to hang with. Im confused about this entire situation. At times yes i want both of them as my woman because we all bond so well together and at times i rather it just be me and my gf but knowing how attached she is to her mom i just cant come to muster the idea of moving into our own again and just living life. The whole crazy thing about it is i get sex once a week. We used to fuck like 3 times a day but literally once a week. I've tried everything but got drained to the point i sit here and say why does good dick have to go to waist. I don't and refuse to cheat on my gf with another women. My mil wants me but is super afraid to admit it. She would buy me stuff and not her own daughter i would eventually tell her its not ok to do that and then she would go eventually get her daughter flowers or something. Im confused and need some help from women and men on this subject.