Where does my dad stand in this divorce situation?

The problem is the answers you are getting stem from America and the situation is different over there. In the UK it is automatically joint property on marriage so she will be entitled to some of the equity in the property however she may not be entitled to it yet, at this point in time are any of you under 16 or under 18 and still in full time education if so she may have to wait till you finish. Also as your father has been paying the mortgage not her that would be taken into account. She would owe back child support but that would be dealt with by the CSA and is completely separate from the divorce, he should already have an order in place for the younger ones, the eldest would not apply at all. Since she has started the divorce proceedings he just needs to respond. Things will be weighted slightly in his favour since he has had the care of the 3 of you for the last 4 years but very soon he will have to find the money to buy her out or sell the property. Don't forget though that the property market is very deflated at the moment so buying her out might not cost as much as you think at the moment. She couldn't have picked a worse time to do this for her. Good Luck