Your own plan

Your mom pulls her hard cock up against her chest the tip between her breasts and pulls her shirt down covering it. You could still see it in her cleavage but she adjusts her breasts a bit and they squeeze over the tip hiding it from view. She then looks back at you now decent "Alright, now how should we convince her to accept"

"Well, she would probably accept or at least try it once if we tell her and I said it was fine. We would have to make sure she enjoyed the first time, so she would be ok with a next time." You spend some time exchanging ideas when a thought occurs to you "Why do we have to tell her at all?"

Your mom pauses as she thinks "You're right since she is moving in I can just trap her in my room and keep dicking her until she completely falls to my cock" Her tone indicates that she was joking but the tip of her cock grew again and was visible in her cleavage so it was at the very least exciting for her.

You fake a cough to break your own imagination away "I was thinking more of either tonight or tomorrow while we are studying you 'accidently' show her your cock. Your size alone will keep her thinking about it until she moves in. Then it should come fairly naturally after that" As long as she wasn't put off by your mom's cock it should work pretty well, you will have to work on her sexual desire. When you brought up having sex she would always gently discourage the idea saying she wasn't interested.

Your mom spent a moment looking at you before saying "You don't even want to tell her that you are ok with it? What are you going to do if she asks you about it?" You didn't really think about that.

Then you just decided to go with your first thought "If she asks I'll just tell her I don't believe her, but I wouldn't want to lie to her so I'll do my best to cut off that conversation" Thinking for a bit you realized that you would have to work with your mom on making her cock as teasing as possible "Well, you can pretend to drop something during dinner having her pick it up and with it up your shirt it wouldn't be all that memorable for her. So why don't we decide how you should place it"

Your mom laughs as she says "You just want to look up my skirt again" But she snaps her fingers at you and points her hand down on the floor in front of her. You take your spot between her now wide open legs and looking up her skirt you can see her sack and the cock going up her shirt.

Your mom in service for you lifts her sack up showing you her wet pussy for a bit while her other hand puts her cock back into her skirt. The fact that it was constantly pointing straight up made the possible positions that it could be in while in her skirt narrowed down to two; Tied down to the Left leg or the Right leg. "Do you have some string or ribbons that match your panties?"

Your mom gets up and returns with a purple ribbon, uncaring about the fact that her cock is pushing up her skirt. She had changed panties to some purple ones that were made up of less fabric. Your mom sits back down and hands you the ribbon. You grab her cock and pull it down, you realize how this must look as you see large glob of precum form. You between her legs pointing her cock at your face, shaking your head you push the cock against the left leg. You secure it with the ribbon. You ended up having to tie it quite tight as your mom's thighs deformed as her cock tried to point back upwards.

You give it a good look and nod your head "You will have to think about something hot when she is under there, the precum will complete the image"

Your mom grins "You mean like how you are helping Ellie cheat on you without her knowing?" You avoid eye contact but you know you look embarrassed. "You don't have to be ashamed, I had a similar situation with a lesbian couple. One of them helped me get their girlfriend in my bed then surprised her by joining in" You almost thought that the ribbon would snap as she speaks

You still can't help but be embarrassed though luckily your mom stands up "I got to get started with dinner, Thanks for your help" She pats your head and walks away, your eyes were locked to her cock that was now well below the skirt luckily the ribbon was tied under the skirt so it wouldn't be noticed unless they were checking her out. So if Ellie notices before she sits down for dinner then your elaborate plan would be doubly effective as she would already have some desire for your mom already.