Why was she refused the visit visa? It could have a bearing on this application depending on the reason(s) she was refused.
Is the immigration lawyer OISC accredited? If not I wouldn't use him/her.
Your SIL will need to show that he has an annual income of £18,600 or above.
She will need a TB test and an English language test. (A1) If her English is pretty good go for the harder tests. B1 is currently required for Indefinite Leave to Remain which is five years further down the line.
Many people on here who have been through it will be happy to help.