There is currently more than £80,000 equity in the property if it was sold immediately
Your sister in law has already received £20,000 and you have offered a further £17,000 - making about £4,000 short of a full 50/50 split - but she has had the benefit of the £20,000 fir the last two years
The fact that she has not lived in the property and you have means that it is not unreasonable that you should have been paying the mortgage without contribution from her.
Arguably if you offered a further £20,000 rather than £17,000 then the offer would be on the generous side of reasonable whilst your current offer is on the low side.
If she still will not agree then the only legal option available is to threaten to apply to the court to force a sale of the property - and claim the costs of the applictaion from her.
Within that application you can negotiate a buy out by you and your husband - but that is negotiable rather than something that the court can Order against the wishes of your sister in law.
I hope that this is of assistance - please ask if you need further details