'Ipar' is the malay word for in-laws, like the sister of your wife is your in-law (ipar). Biras goes a little bit I think (though I may be wrong). An example would be like this, your brother's wife is your sister in law (kakak ipar), but her other siblings and family is your 'biras'. This is because you're not married to that woman (your brother are), so you cannot call them YOUR in-laws, hence the term 'biras'
Other's like 'besan' is usually used (as I often heard) to describe the relationship between the parents of both husband and wife.
Ibu/Bapa Mertua (Mother/Father in law) is used when you are refering to your spouse's parents. But YOUR parents and your SPOUSE's parents refer to each other as 'besan'.
So instead of your mother saying. "I just met my daughter-in law's mother at Starbucks" it can just be "I met my 'besan' at Starbucks", although you definitely could not tell which one she met (the mother or the father of her daughter in law). A little bit confusing but hope it helps.