silverlight 4.0 - How to access a value based on siblings or parent in datagrid databinding -

sorry wordy title - battling work out if problems caused by

1. bad class design (likely)
2. ignorance (very likely)
3. both (most likely)

i have spent ages googling due not understanding problem it's harder find solution.

basically, have "day" class has observablecollection of period called periods. each period has length of time. have method/property of day class (totaltimeupuntilthisperiod) takes in period , returns amount of time of periods until point.

i need put period level though datagrid can bind (it's itemssource set day.periods , it's datacontext set day). however, have calculation in day period instances don't know siblings , feels wrong if child walked tree parent level , summed children.

my thinking - period should have no "state" ie. no idea of in day in relation it's siblings - job of day class.

so, if class design correct - how can access datacontext in bound datacolumn? tried write converter take in period , parent datacontext (day) , call method return string bind @ period level) not figure out how make converter aware of datagrid datacontext , want know if on right track.

this bad design considered replicating parent property (e.g. totaltimeupuntilthisperiod) in child class (period) , somehow setting whenever duration changed bind got messy.

periods can added/removed datagrid/day can't calculated on rowload - has dynamic (i guess use linq sum lengths of previous periods)

so think questions are:

1. is class design above ok? (ie. children should know nothing of parent or fellow siblings?)
2. if so, how bind parent properties or methods in datagrid if value dependent on state of parent?

thanks pointers.

update:

here more things have tried in cold light of day:

1) text= "{binding datacontext.sometextfield, elementname=layoutroot}"

this not work - works fine when outside datagrid not when inside.

2) tried setting datacontext of textbox in templated datagrid column - works well, need pass in row datacontext , parent datacontext converter (in order lookup total time periods before one) , can't figure out how that.

so question may - how pass in the current row datacontext converter , parent datacontext (or "how make converter aware of parent datacontext").

here did in end - love know if practise or not:

i made parent class property on child class (this felt wrong?) - can reference parent's properties in datagrid.