When I sell an investment property with multiple owners, is the capital gain divided among all the owners? i have been claiming all rental income due to simplicity

First of all, you should have been filing Partnership tax returns (Form 1065) all along.  ALL income, expenses & depreciation is entered on the Form 1065.  Then a K-1 for each partner is be generated for reporting of said info on their personal tax returns.

Please see a local professional to straighten all this out before you sell the property.

https://www.irs.gov/businesses/small-businesses-self-employed/partnerships

Really? Because an accountant has done this for us in the past and didn't think form 1065 is needed. Also to make it more complicated, the home is owned by 4 people but the mortgage is on 2 person's name only so only those 2 person can claim the interest expenses.