You should present your case as your wife wanting emotional support or your SIL wanting to meet their new nephew or niece. Should not indicate that your SIL is needed to help your wife. You SIL would also need to show that she has the funds to support her own travels, that she has strong ties to her home country (job, property, spouse and children that will remain in home country, etc), that she has approved leave from work, that she has previous international travel, etc. I would not ask for more than 2-3 weeks given her age and profile. How long a visit did you ask for in the previous TRV application?