you have every right to be concerned about your sister in laws behaviour
You have every right to be concerned about this very bad and inappropriate behaviour on both their paths. Please pluck up your courage and speak about it openly in front of your in laws and husband when you are all together. There is nothing innocent in any woman who deliberately tries to ingratiate herself in the regard of a married man nor is there anything innocent in a married man allowing such attempts to continue. Learn from all the horror stories out there of wives who were left devastated becasue their husbands supposed innocent friendship with another woman lead to him marrying the friend. If your husband has such a poor character you have alot of heartache ahead unless you make it known right now that you will not accept this. Please do not accept it. You deserve better. Anyone who tells you its ok for them to do this is very mistaken.
Shukrun for all the replies
Shukrun for all the replies, this has really helped me to pluck up the courage to confront her.
Shukr it’s been a week now and she has not contacted my husband.
May Allah reward all of you in abundance.