For all practical purpose of this question, consider love between a youngsters.

Lust as understood as the desire for physical/sexual pleasure is a necessary, inseparable ingredient of love. Nature designed it so. With practical wisdom too. No need to shy away from this reality. A car is a wonderful mode of transport. It is operated with the help of many parts. There is brake which is to be used with discretion for a happy journey. Restraint, judicious decision can make the lust ingredient the perfect icing for the cake of love!

Youngsters or Oldies..without lust..love feels empty. How many so called soul mates will last for even a week..let alone a month without this alluring, enticing lust?? Simple truth is..it's possible to enjoy lust with out love. but it's NOT at all possible to enjoy love without lust!

Can we conclude that lust is the only difference in the love of two lovers and love between brothers and sisters?

Yes, not only between sisters and brothers but all persons other the partner of the opposite sex.