You cannot request a visa validity of 10 years. That's something a visa officer will determine.
Also, stating a long stay + intention to visit again in future may not work well.
Are you applying for super visa for her or for a TRV/visitor visa?
Okay. But, still won't be a strong tie as her husband co-owns it.
His income is low.
Proofs of his income from farming? Tax returns?
Okay
Super visa application or TRV?
I didn't get this one. I don't think your MIL needs to accompany her everywhere. That's not what you meant, right?
Is your sister-in-law studying/employed/business?