Both of you want to have sex with her so ask your wife to play the major role in convincing her because she is her mother and your wife knows her very well. Even though if her mother does not like the idea then being her mother, she would not make it a big deal so your wife has to take a lead to make her ready for sex, first lesbian and then threesome. However, if she does not want to have an intimate relationship with either of you then you both should respect her decision and drop the idea of having sex with her but you have every right to take a risk of convincing her because who knows she might want the same.
There are two sides of the problem 1. What the woman wants 2. What is your expectation?
If she has some kind of interest then you have to take the lead and clear the matter. Until and unless you discuss the matter then the chances of solution would be low whereas the opposite would give good result. Your mother in law have wonderful physique that doesnot mean that without her consent you are going to have sex with her with your wife. These type of relationship possible where the sex is the priority and relation doesnot matter.
If she is thirsty for sex then nothing to worry.