I think its normal for grandparents to want to spend time alone with their grandkids, as well.
However, I don't think forcing your son to "get used to her" by leaving them alone together is the best thing. Sometimes we have to do that kind of stuff (ie daycare), but if you don't have to, I don't see the need for it. My MIL was constantly on us to just let DS cry with her so he'd get used to her. To the point of walking away from DH and I during family get togethers when we'd try to take our hysterical 7 mo old away from her. I saw red whenever this happened. I was able to block her on actually leaving him with her as an infant because he was breastfed and refused a bottle. Since I gave him what he needed as an infant (a lot of holding, love, cuddles etc), he's becoming confident and not freaking out about being with anyone who isn't Mommy anymore. (He's only 13 mos old now and somewhere in that not quite an infant but not really a toddler yet stage, but the change in his confidence level of other people is huge.) I think if he's not really ready for it and you don't need to, I don't see a need to force him to get used to her.
Also, you said you work full time so your a little jealous of your weekends. My thought here is that its your DS's weekend with you, too. In his own 2.5 yr old way, he's probably just as jealous of that time and wants to see you.
Instead of forcing him to get used to her by leaving them together, would it be an option to have her come over while you are busy and in another part of the house for an hour? That way she's getting some alone time but you're still there in case your DS needs you. You could set up some ground rules like if he cries for more than five or ten minutes, you'll come in and be with them. Your MIL may not go for that since you'd still be responding to him rather than forcing him to get used to her, but it could be a good middle ground. I'd just prepare myself for eye rolling and you're babying him comments if that's her style.