UANBU - Ask DH to sit her down and ask how will she pay council tax fees, heating, emergency repairs etc for such a big house when she is not working.
Jobs are also even harder to come by with the pandemic. He will need to be honest & tell her the business is at risk of losing staff or even closing down all together, which means you will need at least 6 months to a years worth of money to cover your own bills and expenses if you have to close your business.
I would sit down with your husband and cost out repairs that need doing in your own house, put that money aside, work out yourself how much you will need a year if the business closed (worst case scenario) and put that money aside too as a back up. Think about your pension too.
Then you will know exactly, how much you can actually afford to loan out.
Get SIL to also work out what she can actually afford, to repay, if she is on benefits. Depending on age of child she will also need to consider child care costs if she intends to look for a job.
If you do loan anything, make sure you have a contract of how, when and how much, will be paid back in writing. Also, what will happen if she starts missing payments??
Even with agencies, the work is minimal this year with another possible lock down on the way, she will probably struggle to find a job & even maintain a small house, if she is not working.
Divorce lawyer fees will also need to be taken into account, as well as other fees when buying a house.
I'm all for helping out family, but SIL needs to be realistic and live within her means.