The fact is that unless the OP has been the subject of physical violence, it's unlikely that she will get away with having changed the locks, and her husband would be perfectly entitled to get another locksmith in to change them again, or even break a window to gain access, as he is a JOINT owner. I really do have every sympathy with the OP, and like her, in that situation would feel tempted to get the locks changed, but a simple Google would have told her that this was not a wise thing to do, and while I know it's unlikely she's even thinking straight with everything that's been going on, I feel sure her sister could have found this out.
I just Googled this question:
if you change the locks of a jointly owned home can the other person break in to gain access?
This is the first thing that came up:
Yes, if you jointly own a home, the other owner generally has a legal right to access it. Changing the locks to exclude a co-owner is usually unlawful, and the other person can use a locksmith to enter or, in some cases, use reasonable force to enter without committing a criminal offence.
Key Implications and Actions:
Equal Rights: Both parties possess an equal right to occupy the property, regardless of who moved out, unless an Occupation Order from a court states otherwise.
Legal Action: A locked-out co-owner can legally change the locks back, seek a court injunction to gain entry, or apply for an Occupation Order.
Exceptions: Lock changes are typically only permissible if you fear for your safety (domestic violence/abuse) or have a court order.
Recommendation: Unilaterally changing locks can make a court view you as acting unreasonably, creating unnecessary conflict and legal costs.
If you feel you are at risk of violence or abuse, contact the police. Otherwise, it is highly recommended to seek legal advice on family law from a professional to manage access and property rights correctly.
So if you haven't already OP, please get URGENT legal advice.